Lecture notes: CHEM103 – November 18, 2008

LECTURE OUTLINE:

1. Conversions between mass & moles

a. Molar mass calculations

2. Conversions using solution concentrations

a. “moles per liter” or M (molar, molarity)

b. dilution of solutions

3. Percent composition of solids, liquids, solutions

4. Finally – STOICHIOMETRY

Conversions between mass & moles

conversion of atoms to moles; moles to mass; mass to atoms

Mass of 2.0 moles of Al?

Remember: N_A = 6.022x10²³ mol^-1

and… 1 amu = 1.6606x10^-24 g

Al: 26.98154 amu = ? (g)

1 mole of Al atoms = ? (g)

…so amu = g/mole!

Molar mass of a compound is simply the SUM of all the molar masses of the atoms in it,

multiplying by factors from the subscripts (BALANCING COEFFICIENTS DON’T MATTER)

HCl

NaNO₃

Ca₃(PO₄)₂

examples:

K₂O: molar mass (g/mol)?

How many moles of potassium oxide in 42 grams?

aluminum oxide: molar mass (g/mol)?

How much mass is in 0.4260 moles of Al₂O₃?

Ca₃(PO₄)₂

How many calcium atoms in 25.00 g?

NOTE:

The terms: “atomic mass” = “molecular mass” = “formula mass” can be replaced by “molar mass” to be general

(also, “molecular weight” sometimes used by me – because I’m so old…)

Conversions using solution concentrations

(molar, OR molarity, OR “M” BUT REALLY, moles/liter)

conversions between volume, mass, molarity and moles

NOTE: molar concentration indicated by use of [x] square brackets.

additional NOTE: when doing calculations, ALWAYS convert M to moles/liter;

and do not abbreviate moles as “m” – it will confuse you when you least expect it…

examples:

1 mL of 16 M HCl: how many moles of HCl are in the solution?

1 mole of HCl: how much volume of 16 M solution is needed?

What mass of acid is required to make 25.0 mL of 1.0 M oxalic acid?

(oxalic acid molar mass = 90.03 g/mole)

more examples:

A mass of 0.15 g NaCl is dissolved in 100 mL of water: what is the molarity of the product(s)?

If you have 100 mL of 0.15 M CaCl₂ : how many moles of chloride (Cl-) are in the solution?