INTERPRETING A CHEMICAL EQUATION

A balanced chemical equation describes the proportionate quantities of reactants and products.

Consider the following chemical equation:

2 NO + O_{2} ®
2 NO_{2}

The coefficients in the balanced equation indicate that 2 molecules of NO react with 1 molecule of O_{2}.

Similarly, this equation also states that 2 moles of NO react with 1 mole of O_{2} to yield 2 mole of NO_{2} product.

Interpretation of Chemical Equation Coefficients

For the General Equation 2 A + 3 B ® C + 2 D

The ratio of molecules is 2 : 3 : 1 : 2

The ratio of moles is 2 : 3 : 1 : 2

The ratio of volumes of gas is 2 : 3 : 1 : 2

**Stoichiometry** 1: a branch of science that deals with the application of the laws of definite proportions and of the conservation of matter. 2 : quantitative chemical properties and composition esp. as a factor in processes of chemical or physical change

Some stoichiometric calculations relate a given amount of reactant with an unknown amount of product or vice versa. Or, a given amount of reactant can be related to another reactant. Same goes for two products.

The same method of problem solving is used regardless of the quantity sought.

How many grams of H_{2} is produced when 0.500 g of Al metal react with hydrochloric acid to give aluminum chloride and hydrogen gas ?

2 Al + 6 HCl ®
2AlCl_{3} + 3 H_{2}

From mass Al convert to moles Al, then convert to moles H_{2}, then multiply by M.M. to convert mass H_{2}

LIMITING REAGENT

The reactant that is completely used up in a chemical reaction.

The amount of this reactant limits the amount of new compound that can be formed.

If you have six pieces of bread and four pieces of ham, how many sandwiches can you make?

First, let us write out a chemical equation for the synthesis of a ham sandwich.

For each reactant, calculate the amount of product that can be produced.

From the above calculation we see that the amount of bread 'limits' the amount a sandwiches that can be produced. Thus, bread is the limiting reactant. The same process is followed for calculating the limiting reactant in a chemical reaction.

If 0.654 g of hydrogen react with 0.500 g of oxygen how many grams of water are produced? Which is the limiting reactant hydrogen or oxygen?

2H_{2} + O_{2} ®
2 H_{2}O

For each reactant, calculate the amount of product that can be produced

First, the mass of water based on hydrogen.

Now, the mass of water based on oxygen.

Now one sees that only 0.563 g of water can be produced and oxygen is the limiting reagent.

MOLAR CONCENTRATION

A common expression of solution concentration is Molarity. Molarity (symbol **M**) is the number of moles of a solute dissolved in one liter of solution.

moles of solute = M

liter solution

(section 14.9 in textbook)

What is pH?

The pH value expresses the acidity of a solution.

- Measure of the hydrogen ion (H
^{+}) conc. - Based on moles/liter
- A logarithmic scale

[H+] = 10^{-pH}

pH = - log[H^{+}]

If 0.365 grams of HCl is dissolved in 1 liter of water, what will the pH be?

HCl ®
H^{+} + Cl^{-} (Strong acid, completely disassociates)

First, find moles HCl

Then find molar conc.

[H+] = 0.01 moles/1 liter = 0.01 M

pH = -log(0.01) = 2