# Chem Talk Chp 4 Section 3

Chemists use moles to count very large quantities of very small objects. One mole is always the same quantity. A mole is similar to the word “dozen”, we use these words to count groups of things. Moles are usually used to count atoms and molecules. The exact number a mole represents is 6.022 * 10^23 units.

There is also the molar mass. The molar mass is the mass of one mole of a pure substance. For example, oxygen’s atomic mass 16.00 amu therefore one mole of oxygen atoms has a mass of 16.00 grams (16.00 grams is the molar mass of oxygen atoms).

To find the mass of a compound, you have to add the masses of each atom in the compound together. For example H2O, H has an atomic mass of 1.01 and oxygen has an atomic mass of 16. When you add all of those together (1.01 + 1.01 + 16.00), you get the mass of one mole of water which is 18.02 grams. (Also you add two hydrogen masses because there are 2 H atoms in H2O)

One mole of almost any gas at standard temperature and pressure (STP) will occupy the same volume (22.4L) — (Standard pressure = 760 mm Hg)–(Standard temperature = 273.15 kelvins). Standard pressure is close to the pressure under which we live. When the temperature of a gas increases, the volume occupied by the gas increases. One mole of a gas is normally (at standard conditions) occupying 22.4L however one mole of a gas at room temperature will be larger.

There is the study of the relationships between mass-mole-volume among substances in chemical reactions that is called stoichiometry. When you are calculating the volume you need to calculate the number of moles of one chemical based on the number of moles of one of the other chemicals in the reaction. The coefficients in the balanced equation relate the number of moles of any reactant or product to the other reactant or product.

To solve a stoichiometry problem you need to know what measurement you want to start and end with. You set the problem up using dominoes. There are three kinds of dominoes:

1. You can use the molar mass of a substance. One mole of the substance and its molar mass.
2. You can use the coefficients from a balanced chemical equation. (both units in moles)
3. You can use the volume that one mole of gas occupies. This will always be the same domino: 1 mol of gas at STP is = to 22.4 L of gas.

To actually solve these problems you use dimensional analysis. This method involves canceling units. You convert from on the bottom of the ratio so that it will cancel the chemical and measure on the top of the preceding ratio. Chemists also use something called percent yield. When a reaction takes place the product recovered is normally less than 100 percent of the expected product. Percent yield is a ratio of the product recovered and the product expected. To find the percent yield you put the volume found over the volume expected and multiply it by 100.