Molar mass
• The mass of one mole is called “molar mass”
• E.g. 1 mol Li = 6.94 g Li
• This is expressed as 6.94 g/mol
• Calculate molar mass to 2 decimal places
What are the following molar masses of each mol of sample?
(a) Sulfur Atom Ms =32.06 g/mol
(b) Oxygen gas MO2 = MO x 2 = 16.00g/mole x 2 = 32.00 g/mole
(c) SO2 Ms + M0 x 2= 64.06 g/mol
(d) Calcium Phosphate MCa x 3 = 40.08 g/mol x 3 = 120.24 g/mol
MP x 2 = 30.97 g/mol x 2 = 61.94 g/mol
MO x 8 = 16.00 g/mol x 8 = 128.00 g/mol
MCa3(PO4)2 = 120.24 g/mol + 61.94 g/mol + 128.00g/mol = 310.18 g/mol
(e) Cu3(BO3)2 308.27 g/mol
Cu Mcu x 3 = 63.55 g/mol x 3 = 190.65 g/mol
B MB x 2 = 10.81g/mol x 2 = 21.62 g/mol
O Mo x 6 = 16.00g/mol x 6 = 96.00 g/mol
MCu3(BO3)2 =190.65 g/mol +21.62 g/mol+ 96.00 g/mol= 308.27 g/mol
(e) (NH4)2CO3 96.11 g/mol (N x 2, H x 8, C x 1, O x 3)
(f) FeSO4·7H2O
Converting Between grams and Moles
• If we are given the # of grams of a compound we can determine the # of moles, & vise-versa
• In order to convert from one to the other you must first calculate molar mass
g = mol x g/mol
mol = g ¸ g/mol
• This can be represented in an “equation triangle”
Example 1: A flask contains 0.750 mol of carbon dioxide gas, CO2. What mass of carbon dioxide gas is in this sample?
Step 1: Write down the appropriate formula. Since you are looking for mass from moles, you will use: m = n x M
Step 2: Givens and unknowns: Given: n = 0.750 mole, MCO2 = 44.01 g/mole (through calculations), Unknown: m = ?
Step 3: Plug givens into the formula (m = n x M):
m = 0.750 mole x 44.01 _g_ = 33.0075 g
mole
Step 4: Statement: Therefore the mass of 0.750 moles of CO2 is 33.0075 g.
Example 2: How many moles of acetic acid, CH3COOH, are in a 0.0236 kg sample?
Step 1: Equation/formula: Since we are looking for n and we have m, the formula is n= m/M
Step 2: Given and Unknowns:
Given: m = 0.0236 kg MCH3COOH = 60.06 g/mol (determined through calculation), Unknown: n = ?
Before you proceed, you must convert mass into grams: 0.0236 kg x 1000 _g_ = 23.6 g
kg
Step 3: Plug in givens into the formula (n = m/M):
n = 23.6 g / 60.06 _g_ = 0.3929 mole
mole
Step 4: Statement:
Therefore, there are 0.3929 moles of acetic acid in 23.6 g of acetic acid.