The formula mass of a substance is the sum of the average atomic masses of each atom represented in the chemical formula and is expressed in atomic mass units. The formula mass of a covalent compound is also called the molecular mass. A convenient amount unit for expressing very large numbers of atoms or molecules is the mole. Experimental measurements have determined the number of entities composing 1 mole of substance to be 6.022 × 1023, a quantity called Avogadro's number. The mass in grams of 1 mole of substance is its molar mass.
Due to the use of the same reference substance in defining the atomic mass unit and the mole, the formula mass and molar mass (g/mol) for any substance are numerically equivalent . The molar mass of any substance is numerically equivalent to its atomic or formula weight in amu. Per the amu definition, a single 12C atom weighs 12 amu .
A mole of 12C weighs 12 g (its molar mass is 12 g/mol). This relationship holds for all elements, since their atomic masses are measured relative to that of the amu-reference substance, 12C. Extending this principle, the molar mass of a compound in grams is likewise numerically equivalent to its formula mass in amu (Figure 3.6). Gas specific gravity is defined as the ratio of the density of the gas to the density of air at 1atm pressure at 60°F . If ideal gas law behavior is assumed, gas specific gravity is the molecular weight of the gas divided by the molecular weight of air. Because natural gas is a mixture, the molecular weight used is a weighted average molecular weight based on the components of the gas.
The identity of a substance is defined not only by the types of atoms or ions it contains, but by the quantity of each type of atom or ion. For example, water, H2O, and hydrogen peroxide, H2O2, are alike in that their respective molecules are composed of hydrogen and oxygen atoms. However, because a hydrogen peroxide molecule contains two oxygen atoms, as opposed to the water molecule, which has only one, the two substances exhibit very different properties. This experimental approach required the introduction of a new unit for amount of substances, the mole, which remains indispensable in modern chemical science. To have value, measurement results must be metrologically traceable to an appropriate reference, which in the cases treated in this chapter are SI units of mass, volume, and amount of substance. A statement of measurement uncertainty always accompanies a traceable result.
Methods must be validated and verified for use by a particular operator at a particular time. Accreditation to an appropriate standard, such as ISO , is overseen by organisations usually with governmental or quasi-governmental status. Gaining accreditation for a particular method shows that a laboratory is using validated methods by competent personnel, but of course can never guarantee a reliable result . In this section we will review the components of measurement uncertainty of mass and volume measurements and then apply this to the preparation of a standard solution and a typical titration.
It is noted that the metrological traceability chain will involve multiple branches , often through amount fraction or mass fraction. For more information, see the chapter on quality assurance in the forthcoming 4th edition of the Orange Book , or in . The relationships between formula mass, the mole, and Avogadro's number can be applied to compute various quantities that describe the composition of substances and compounds.
For example, if we know the mass and chemical composition of a substance, we can determine the number of moles and calculate number of atoms or molecules in the sample. Likewise, if we know the number of moles of a substance, we can derive the number of atoms or molecules and calculate the substance's mass. Molar mass is equal to the mass of one mole of a particular element or compound; as such, molar masses are expressed in units of grams per mole (g mol–1) and are often referred to as molecular weights. The molar mass of a particular gas is therefore equal to the mass of a single particle of that gas multiplied by Avogadro's number (6.02 x 1023 ).
To find the molar mass of a mixture of gases, you need to take into account the molar mass of each gas in the mixture, as well as their relative proportion. Many argue that modern chemical science began when scientists started exploring the quantitative as well as the qualitative aspects of chemistry. For example, Dalton's atomic theory was an attempt to explain the results of measurements that allowed him to calculate the relative masses of elements combined in various compounds. Understanding the relationship between the masses of atoms and the chemical formulas of compounds allows us to quantitatively describe the composition of substances. For more accurate measurements, glassware that has been certified by standards agencies may be purchased. The glass volumes are also calculated for the standard temperature of 20 °C, with small adjustments for borosilicate glass expansion or contraction with temperature changes.
We can argue that modern chemical science began when scientists started exploring the quantitative as well as the qualitative aspects of chemistry. Is the volume occupied by one mole of a chemical element or a chemical compound. It can be calculated by dividing the molar mass by mass density (ρ). Molar gas volume is one mole of any gas at a specific temperature and pressure has a fixed volume. Volume is an amount of space, in three dimensions, that a sample of matter occupies.
The number and the phase of the molecules in the sample primarily determine the volume of a substance. Volume will be measured in many ways in this course, but the units are usually milliliters or cubic centimeters . Methods for determining or delivering precise volumes include volumetric pipets and pycnometers; less precise methods include burets, graduated cylinders, and graduated pipets. The measurement of mass is a central point of the quantification of material substances. A balance measures mass by sensing the weight force that presses an object down on the balance pan.
Weight is the force exerted on a body by the gravitational field of the earth, and is measured in the unit force newton, N. The weight force acting on 1 kg mass depends on geographic and cosmic factors. However, for mass measurements using mechanical balances, the weight of the unknown object is equilibrated at the same place and same time as the weight of an object of known mass (i.e. of a standard). For high precision measurements, the buoyancy caused by the surrounding air must be taken into consideration. This correction can easily be calculated if the density of the known and unknown mass and that of the air is known.
The mole is an amount unit similar to familiar units like pair, dozen, gross, etc. It provides a specific measure of the number of atoms or molecules in a bulk sample of matter. A mole is defined as the amount of substance containing the same number of discrete entities as the number of atoms in a sample of pure 12C weighing exactly 12 g. One Latin connotation for the word "mole" is "large mass" or "bulk," which is consistent with its use as the name for this unit. The mole provides a link between an easily measured macroscopic property, bulk mass, and an extremely important fundamental property, number of atoms, molecules, and so forth. In an earlier chapter, we described the development of the atomic mass unit, the concept of average atomic masses, and the use of chemical formulas to represent the elemental makeup of substances.
These ideas can be extended to calculate the formula mass of a substance by summing the average atomic masses of all the atoms represented in the substance's formula. MetalProject the image MetalMost common metals like aluminum, copper, and iron are more dense than plastic or wood. The atoms that make up metals are generally heavier than the atoms in plastic and wood and they are packed closer together. Plastics are made from individual molecules bonded together into long chains called polymers. These polymer chains are arranged and packed together to make the plastic.
One common plastic, polyethylene, is made up of many individual molecules called ethylene which bonded together to make the long polymer chains. Like most plastics, the polymers in polyethylene are made of carbon and hydrogen atoms. The carbon and hydrogen atoms are very light, which helps give plastics their relatively low density.
Formula For Volume Chemistry Density Plastics can have different densities because different atoms can be attached to the carbon-hydrogen chains. The density of different plastics also depends on the closeness of packing of these polymer chains. WoodProject the image WoodWood is made mostly from carbon, hydrogen, and oxygen atoms bonded together into a molecule called glucose. These glucose molecules are bonded together to form long chains called cellulose. Many cellulose molecules stacked together give wood its structure and density.In general, the density of wood and plastic are similar because they are made of similar atoms arranged in long chains.
The difference in density is mostly based on the arrangement and packing of the polymer chains. Also, since wood is from a living thing, its density is affected by the structure of plant cells and other substances that make up wood. The mass of a mole of any solid will be equal to the sum of the relative atomic masses that make up the chemical. You may often hear your teacher or classmates refer to this as the molecular or molar mass.
The relative formula mass of a compound is calculated by adding together the relative atomic mass values for all the atoms in its formula. While atomic mass and molar mass are numerically equivalent, keep in mind that they are vastly different in terms of scale, as represented by the vast difference in the magnitudes of their respective units . To appreciate the enormity of the mole, consider a small drop of water weighing about 0.03 g (see Figure 3.7). Although this represents just a tiny fraction of 1 mole of water (~18 g), it contains more water molecules than can be clearly imagined.
If the molecules were distributed equally among the roughly seven billion people on earth, each person would receive more than 100 billion molecules. Recall that the number of moles, n, is equal to the mass of the gas divided by its molar mass. Substituting this relationship into the ideal gas equation, and then rearranging, yields an expression for mass over volume or density.
Add the atomic masses of the solute together to find the molar mass. Look at the elements in the chemical formula for the solute you're using. List the atomic mass for each element in the solute since atomic and molar mass are the same. Add together the atomic masses from your solute to find the total molar mass.
Calculate the uncertainty in the mass of water removed using error propagation. Convert this mass to volume units by dividing by the density of water (use a precise value, specific to the water's temperature). This value equals the uncertainity in the volume of the metal cylinder. Matter is made up of atoms and molecules, and the more of them we have, the greater the mass of an object is.
We measure mass in units like kilograms, ounces, and pounds. In science, we prefer kilograms, which are the SI unit of mass. Your bathroom scale is calibrated to tell you your mass, but it only works properly on Earth.
If you took it to the moon, the weight on the scale would change, but of course, your mass would not. Similar terms apply to complexometry , oxidation-reduction, and precipitation titrimetry. In the last case, substances which are adsorbed or desorbed, with concomitant colour changes at or near the equivalence-point, are termed adsorption indicators.
It is usually expressed in terms of the negative decadic logarithm of the concentration (e.g. pH, pM) or, for oxidation-reduction titrations, in terms of a potential difference. Standard solutions are often prepared by dissolving an accurately measured mass of a solute of certified purity in a known volume of solvent. If the concentration of an intended standard solution is obtained by measurement, for example by titration with a standard solution, it is known as a secondary standard [VIM 5.5]. Use the molecular formula to find the molar mass; to obtain the number of moles, divide the mass of compound by the molar mass of the compound expressed in grams. Consistent with its definition as an amount unit, 1 mole of any element contains the same number of atoms as 1 mole of any other element. The masses of 1 mole of different elements, however, are different, since the masses of the individual atoms are drastically different.
The molar mass of an element is the mass in grams of 1 mole of that substance, a property expressed in units of grams per mole (g/mol) . Because the density of water in g/cm3 is 1.0, the SG of an object is will be almost the same as its density in g/cm3. However, specific gravity is a unitless number, and is the same in the metric system or any other measurement system. It is very useful when comparing the density of two objects. Since specific gravity is unitless, it doesn't matter whether the density was measured in g/cm3 or in some other units (like lbs/ft3).
Water hardness, the concentration of titratable calcium and magnesium, is measured by complexiometric titration using the blue dye Eriochrome Black T as the indicator. When added to water, the indicator reacts with Ca2+ and Mg2+, exhibiting a wine-red colour. Upon reaction with the titrant EDTA the colour becomes blue. Water hardness is expressed as an amount concentration of calcium and magnesium or an equivalent mass concentration of calcium carbonate or calcium oxide. End-point error – the systematic error occurring because the equivalence-point potential differs from the end-point potential under the given conditions of titration.
The equivalence-point potential depends on the formal potentials of the analyte and titrant and on the number of electrons participating in half-reactions. When the transition potential, corresponding to the end-point, is close to the equivalence-point potential, the effect of the above-mentioned factors may be diminished. Transition potential is often given instead of the formal redox potential. It corresponds to the colour change at which the end-point is said to occur.
It is a function of the formal redox potential, the total concentration of the indicator , the depth of the colour layer, the minimal observable absorbance , and the absorption coefficient. In an ideal two-colour indicator, the "apparent absorption coefficients" of both forms should be equal. Then the transition potential approaches the formal one. As for formal redox potential, it should be given, at least for the acidity range of indicator application. The transition potential may be given for pseudo-reversible indicators. Because the transition point is usually different for oxidimetric and reductiometric titrations, it is sometimes useful to distinguish those two values.
(Add the atomic masses of the constituent elements.) Then, convert milligrams to grams by dividing by 1000. Finally, divide the grams of your substance by the Molar Mass. Students will be able to calculate the density of different cubes and use these values to identify the substance each cube is made of.
Students will be able to explain that the size, mass, and arrangement of the atoms or molecules of a substance determines its density. Two solutions that have the same molarity will have the same number of molecules of the chemical per liter but are likely to contain differing masses of that chemical per liter to achieve this. Whereas two solutions at the same concentration will have the same mass of the chemical per liter of solution but are therefore likely to have differing numbers of molecules of that chemical per liter.
Provided some additional information is known, one value can be deduced from the other using the equations below. In solutions, mass concentration is commonly encountered as the ratio of mass/, or m/v. In water solutions containing relatively small quantities of dissolved solute , such figures may be "percentivized" by multiplying by 100 a ratio of grams solute per mL solution. Such a convention expresses mass concentration of 1 gram of solute in 100 mL of solution, as "1 m/v %". Water displacement method was invented by Archimedes.
