Chemistry Stoichiometry Test

Gram Formula Mass

the formula mass expressed in grams instead of atomic mass units

Molar Mass

the mass of a mole in a substance

Mole

the number of atoms in carbon in 12.000g of C-12, 6.02 × 10²³

Avogadro’s Number

the number of particles in a mole (6.022 × 10²³)

Percent Composition by Mass

the concentration of a solution expressed as the ratio between the mass of the solute and the total mass of the compound

Hydrate

a crystalline ionic substance that contains a definite amount of water molecules physically associated with it

Anhydrous Salt

a hydrate that has been dehydrated

Mole Ratio

the ratio of coefficients in a chemical reaction

Conversion Factor

a ratio equivalent measurements used to convert a quantity from one unit to another

Stoichiometry

calculations of reactants or products in a chemical reaction based on the Law of Conservation

the relationship between the quantities of reactants and products before, during, and after chemical reactions

Molecular Formula

the actual ratio of atoms in a molecule

Empirical Formula

the simplest integer ratio of atoms bonded in a compound; the skeleton formula

How can water be removed from a hydrate?

by heating

Why can water be removed from a hydrate by a physical change?

because the water is physically associated, not chemically

What is the remaining substance called when water has been evaporated from a hydrate?

anhydrous (an anhydrate)

How can you determine how many grams of water are removed from a hydrate?

by using the percent mass equation, using the ratio to find exactly how many grams would be lost when heated

Why do some hydrates experience a color change when drying?

Water can become trapped underneath crystals and therefore has difficulty evaporating

What kind of change is a color change in hydrates?

a physical change

When can you be sure that you’ve totally dried a sample of a hydrate?

when the mass is constant

How do you name a hydrate?

1) name the ionic part following regular ionic naming rules

2) the number of water molecules becomes a covalent prefix for “hydrate”

How do you convert moles to grams and vice versa?

Find the number of grams per mole with a QMT chart and multiply/divide so that the units you don’t want cancel out

What do coefficients in chemical equations tell us?

the ratio of one substance to another

What could coefficients represent?

• moles of a substance

• numbers of particles

• liters of gases at STP

What is the word ratio for N₂ + 3H₂ —> 2NH₃?

Every single mole of nitrogen requires three moles of hydrogen to make 2 moles of ammonia

Sometimes the amount of substance you want or need doesn’t always match the ratio given by the chemical substance. How can you fix this?

you can use a balanced equation to help you translate the amount of reactants you’ll need. Use the coefficients as conversion factors

What do moles represent?

a quantity of atoms or molecules

What must you use to determine how much substance you’re working with?

grams

What is 1 mole of any gas at STP?

22.4 liters

How many molecules are in 1 mole?

6.022 × 10²³ molecules

How do you format the conversion factor from a balanced equation in a stoichiometry equation?

mol unknown/mol given

the number of moles of the substance you’re trying to find/the number of moles of the given substance

How do you get the empirical formula from a molecular formula?

you reduce the subscripts as much as possible to get their simplest ratios

How do you get the molecular formula when given an empirical formula?

find some multiple of the original ratios

What pieces of informaiton are you usually given when trying to find a molecular formula?

the ratio of the elements and the molecular formula’s mass or the empirical formula

What should you do when trying to find a formula and you’re given percents?

assume the sample is 100g and convert the percents to grams (ex. 50%=50 grams)

What are the steps of going from a percent to empirical equation?

1. assume sample is 100g, convert percentages into grams

2. convert grams into moles

3. divide all moles by the smallest mole value from step 2 to get whole numbers

4. any time you end up with anything x.5, double everything to get rid of the half

What are the steps of going from a percent to molecular equation?

1. assume sample is 100g, convert percentages into grams

2. convert grams into moles

3. divide all moles by the smallest mole value from step 2 to get whole numbers

4. any time you end up with anything x.5, double everything to get rid of the half

5. do a QMT chart to find the mass of the empirical formula. Divide the molecular mass by the empirical mass (given) to find the multiple used to get your formula

6. multiply empirical subscripts by multiple found in step 5

What is a limiting reactant?

a reactant that is used up first, therefore determining how much product can be formed

What should you always use the limiting reactant for?

calculating the amount of the product

What is an excess reactant?

the reactant that is not used completely and there is some left over

When do you determine the limiting reactant?

when the problem gives the amount of both reactants

When do you not determine the limiting reactant?

when one reactant is “in excess” or just one amount is given

What is the actual yield?

the amount of product produced by a reaction (in the lab)

Where do you get actual yield from?

either it’s given in the problem or determined from experimental data

What is the theoretical yield?

the maximum amount of product that can be produced from given amounts of reactants

What is percent yield used for?

determining the efficiency of a reaction