From WolfWikis

Chapter 1 Worksheet

We will use the unit factor method for conversions. The key is to set up your equation so that the conversion factor cancels the unwanted units and you are left with the desired units.

Example I:

Q: How many moles of CaCO3 are there in 5.36g CaCO3?

A: We are asked to convert mass to moles which means we need to use the molar mass of CaCO3 in g/mol as the conversion factor. The unwanted units are grams (mass) and desired units are moles so we will multiply the mass by the reciprocal of the molar mass. So first we calculate the molar mass of CaCO3, and then set up the equation so that unwanted units cancel.

Mm (CaCO3) = 100g/mol

Practice I:

Q: What is the mass of 0.31 mol CaCO3?

Hint: Now we are asked to convert mass to moles, opposite of the example above. Set up

your conversion so that the unwanted units (mol) cancel.

Example II:

Q: How many CaCO3 particles are there in 12.6g of CaCO3?

A: Here we are asked to find the number of particles which means that we need to use Avogadro’s #. However, we can only use NA as a conversion factor when working with moles so we first need to convert grams of CaCO3 to moles of the same.

Practice II:

Q: What is the mass of 2.4x1020 particles of CaCO3?

Hint: Now we are asked to convert the number of particles to grams of substance so we use Avogadro’s number but we are converting in the opposite direction. Make sure you set up the conversion so that units of particles cancel and you’re left with the units in grams.

Stoichiometry

Chemical formulas as well as chemical reactions are always shown having a specific ratio of atoms.

Formula stoichiometry:

Example III:

Q: How many moles of oxygen atoms are present in 5.36g of CaCO3?

A: First we need to calculate the number of moles of CaCO3 in 5.36g CaCO3. We already calculated that to be 0.0536 in Example I. We now use the ratio of the number of moles of O in each mole of CaCO3 as the conversion factor.

Example IV:

Q: How many oxygen atoms are there in 0.0536 mol CaCO3? A: First we must calculate the moles of oxygen atoms as above, than use Avogadro’s # to calculate the number of oxygen atoms.

Practice III:

Q: How many moles of atoms are there in 1.7g of CCl4?

Reaction stoichiometry:

Balanced chemical equations tell us something about the relative ratios of reactants and products for that particular reaction. In the reaction above 1mol of CH4 reacts with 2mol of O2 to form 1mol of CO2 and 2mol of H2O. Regardless of how many moles of reactant you actually start with, the ratio of reactants and products remains constant. We can use ratios of the coefficients as conversion factors when converting from one substance to another.

Example V:

Q: How many moles of CH4 are required to react with 0.5 mol O2 to form products?

A: Our conversion factor is the ratio of the coefficients in front of CH4 and O2.

Practice IV:

Q: How many moles of H2O are formed if 4.3 g of O2 reacts completely?

Hint: There is an extra step in this problem. First you must convert grams of O2 to moles of O2 then use the ratio of coefficients as the conversion factor.

Limiting reactant:

Chemists seldom add reactants in the exact stoichiometric ratio. Usually one of the reactants is added in excess which means that only one reactant will react completely, where some of the other reactant will remain unreacted. The reactant that reacts completely is called the limiting reagent.

Example VI:

Q: How much Al2O3 can be produced from 4mol Al and 6mol O2?

Balanced chemical equation: 4Al + 3O₂ => 2Al₂O₃

A: To find limiting reactant we must find out how much Al2O3 can be made from each reactant. We calculate this by using reaction stoichiometry as shown in Example V.

Practice V:

Q: Consider the reaction of 3mol P4 and 10mol O₂ to produce P₂O₅.

a) Write the balanced chemical reaction

b) How many moles of P₂O₅ could be produced from the reaction of 3mol P₄ and 10mol O₂?

c) How many moles of excess reactant remain after the reaction is done?

Extra practice problems in Ch1: 5, 7, 9, 15, 19, 21, 23, 25, 29, 35, 45, 47, 53