Download 18.0 g/mol

Find the % by mass of oxygen in
water
• Two things you need to know
H2O
– Mass of part
– Mass of whole
2 atoms of hydrogen
2 x 1.0 g/mol
2.0 g/mol hydrogen
1 atom of oxygen
1 x 16.0 g/mol
16.0 g/mol oxygen
2.0 g/mol + 16.0 g/mol = 18.0 g/mol for H2O
16.0 g/mol
18.0 g/mol
x 100% =
88.9% oxygen
Episode 702
Lab Excerise
• We are going to react sodium bicarbonate with
hydrochloric acid
NaHCO3 + HCl  NaCl + CO2 + H2O
1. Measure and determine the mass of CO2
produced in the reaction
2. Determine the % carbon in the CO2 produced
3. Determine the % carbon in the NaHCO3 used
Episode 702
Find the % carbon in sodium
bicarbonate (NaHCO3)
Remember:
(mass of element in 1 mol of compound ÷ molar mass of compound) x 100%
Na
H
C
O
1 x 23 g/mol = 23 g/mol
1 x 1 g/mol = 1 g/mol
1 x 12 g/mol = 12 g/mol
3 x 16 g/mol = 48 g/mol
84 g/mol NaHCO3
12 g/mol
84 g/mol
x 100% =
14.3% carbon
Episode 702
Find the % composition of
aluminum oxide
• Write formula
Al2O3
• Calculate molar mass
Al
O
54 g/mol
102 g/mol
2 x 27 g/mol =
3 x 16 g/mol =
x 100% =
48 g/mol x 100% =
102 g/mol
54 g/mol
48 g/mol
102 g/mol
52.9% aluminum
47.1% oxygen
Episode 702
Empirical Formula
• Simplest whole number ratio of elements in a
compound
– Example
• Find the empirical formula for a compound
containing 56.6g K, 8.7g C, and 34.7g O
Step 1: Convert each mass into moles of the element.
56.6g K
1mol K = 1.45 mol K
39.1g K
8.7g C
1mol C
= 0.73 mol C
12g C
34.7gO
1mol O = 2.17 mol O
16gO
K1.45C0.73O2.17
Step 2: Divide by the smallest to
find a simple whole number.
K2CO3
Episode 702
32.37% Na
22.64% S
44.99% O
100 %
32.37g Na
22.64g S
44.99g O
100 g
1.41 mol Na
0.705 mol S
2.81 mol O
(Hint: When % are given, assume you have 100g of the
compound, and the % changes to grams.)
Step 1: Convert each mass into moles of the element.
Na1.41S0.705O2.81
Step 2: Divide by the smallest to find a simple whole number.
Na2SO4
Episode 702
PxOy
Determine the grams of
oxygen in the sample.
10.15 g sample
10.15g – 4.433g = 5.717g oxygen
4.433 g P
Step 1: Convert each mass into moles of the element.
4.433g P
1 mol P
= 0.143 mol P
31g P
5.717g O 1mol O
= 0.357 mol O
16g O
P0.143O0.357
Step 2: Divide by the smallest to find a simple whole number.
PO2.5
(Hint: If the ratio is still not whole numbers, multiply both subscripts
by a number, such as “2” to get rid of fraction.)
P2O5
Episode 702