Salts can enter waterways by both naturally and via human activity.

Human activity, and to a lesser extent natural processes, causes some of the following to enter our waterways:

Human activity, and to a lesser extent natural processes, causes some of the following to enter our waterways:

- Heavy metals such as lead, copper, mercury, cadmium. Heavy metals can bioaccumulate in food chains.
- Metals that cause hard water- such as calcium, magnesium and iron.

**BALANCED EQUATIONS**

•The study of relative amounts (ratios) of substances involved in chemical reactions is known as stoichiometry.

The ratio of reactants and products never changes for a particular reaction

·

__Example:__Nitrogen reacts with hydrogen to produce ammonia according to the following equation:

N2 (g) + 3H2 (g) → 2NH3 (g)

__Ratios__

Moles: 1 3 2

Mass (g): 28 6 34

Particles: 6.02 x 1023 1.8 x 1024 1.2 x 1024

(Molecules)

**MASS-MASS STOICHIOMETRY**

•Calculations involving a balanced equation to calculate a quantity (how much?) of a particular chemical.

•Eg: mass, volume, concentration, number of particles, etc…

__4 Step Process__:

1. *Write a balanced equation for the chemical reaction.

*Identify the known (given) substance and the unknown (required) substance.

*Write given data under relevant chemical.

2. *Calculate n (known).

3. *From balanced equation, find the molar ratio of unknown to known substances.

*That is, n (unknown) = ? x n (known)

*Note: Use n (known) calculated in Step 2.

4. *Answer the Question.

*Eg: Calculate mass, concentration, volume, number of particles, etc…

*Note: Use n (known) calculated in Step 3.

•Calculations involving mass of reactant or product.

•Formulas to use:

n = m / M

or

m = n x M

__Sample Problem__:

•Some sulfur is present in coal in the form of pyrite, FeS2. When pyrite burns, it pollutes the air with one of its combustion products, sulfur dioxide, and produces solid iron (III) oxide.

•What mass of iron (III) oxide is formed from the complete combustion of 183.5 g of pyrite?

__Solution__:

Step 1: Write a balanced equation and identify known and unknown.

4 FeS2 (s) +11 O2 (g) → 2Fe2O3 (s) + 8SO2 (g)

Known unknown

183.5 g

Step 2: Calculate n (known).

n(FeS2) = m / M

= 183.5 / (55.85 + 2 x 32.1)

= 183.5 / 120.05

= 1.530 mol

Step 3: From equation find ratio of n(unknown) to n(known).

n(Fe2O3) = 2/4 x n(FeS2)

= 2/4 x 1.530

= 0.764 mol

Step 4: Answer Question

Find mass of iron oxide.

m (Fe2O3) = n x M

= 0.764 x (2 x 55.85 + 3 x 16) = 0.764 x 159.7

= 122 g