Post on 28-Dec-2015
DO NOW 3/5/14
What is the molar mass of water?
What % of the mass comes from hydrogen?
What % of the mass comes from oxygen?
MOLAR MASS OF COMPOUNDS
Add up the atomic masses of all the atoms in the formula.
2 mol H x 1.01 g/mol = 2.02 g H1 mol O x 16.00 g/mol = 16.00 g OH2O molar mass 18.02 g/mol
MOLAR MASS OF COMPOUNDS
Assume 1 mole of the compound
Divide each element mass by total
2.02 g H18.02 g H2O
16.00 g O18.02 g H2O
0.112 x 100% = 11.2% H
0.888 x 100% = 88.8% O
CALCULATE MOLAR MASS FOR
CH3CH3
Fe2(SO4)3
Then name them.
6 mol X 1.01 g/mol = 6.06 g+ 2 mol X 12.01 g/mol = 24.02 g 30.08 g/mol
2 mol X 55.85 g/mol = 111.7 g+ 3 mol X 32.06 g/mol = 96.18 g+12 mol X 16.00 g/mol = 192.00 g 399.88 g/mol
Dicarbon hexahydride(ethane)
Iron (III) Sulfate
CALCULATE PERCENT COMPOSITION FOR
CH3CH3
6.06 g H out of 30.08 g CH3CH3
= 6.06/30.08 = 20.15 % H
24.02 g C’s out of 30.08 g CH3CH3
= 24.02/30.08 = 79.85 % C
DO NOW;:
Pick up a knowledge organizer and a textbook
What is the % composition of ammonium phosphate?
DETERMINING A COMPOUND FORMULA
Empirical means, ‘by experiment.’
The empirical formula is usually the simplest mole ratio of elements, such as 1H:1O or HO.
So the simplest mole ratio gives us the empirical formula.
DETERMINING EMPIRICAL FORMULAS
1. Determine how many grams of each element are present in the sample.
2. Determine how many moles of each element are present.
3. Divide all the moles by the smallest number of moles to get the mole ratios.
4. Complete the formula using the mole ratios
DETERMINING A COMPOUND FORMULA
A substance contains 2 g of hydrogen and 32 g of oxygen. 1. How many moles of each?2. What is the ratio of moles H to
moles O?3. What is the empirical formula?
HxOy (what are x and y?)
2 g H x 1 mole H = 2 moles H 1 g H
32 g O x 1 mole O = 2 mole O 16 g O
Mole ratio = 2:2 or 1:1Possible formulas = HO, H2O2, H3O3
Empirical formula = HO
What is the empirical formula for a substance with a mass
composition of:92. 3 g copper, 48.2 g sulfur,
and 96.0 g oxygen
What is the empirical formula for a substance with a mass
composition of:92. 3 g copper, 48.2 g sulfur,
and 96.0 g oxygen
What is the formula for a substance with a mass composition of:
92.3 g copper x 1 mole Cu = 1.5 mol Cu
63.5 g Cu48.2 g sulfur x 1 mole S = 1.5 mol S
32.1 g S96.0 g oxygen x 1 mole O = 6 mol O
16.0 g O
What is the formula for a substance with a mass composition of:
1.5 mol Cu = 1 mol Cu1.5
1.5 mol S = 1 mol S1.5 6 mol oxygen = 4 mol O1.5
Empirical formula CuSO4
Empirical formula CuxSyOz
Now you try it. What is the empirical formula for a substance with a mass composition of:
36.8 g of oxygenand 13.8 g carbon?
36.8 g O x 1 mole O = 2.3 moles O
16.00 g O
13.8 g C x 1 mole C = 1.15 mole C
12.01 g C
= 2 1.15
= 11.15
Empirical Formula: CO2
The percent composition of a substance is 78.1% B and 21.9% H.
What is the empirical formula for this substance?
Assume you have 100g of the substance.
78.1 g B x 1 mole B = 7.23 moles B
10.8 g B
21.9 g H x 1 mole H = 21.9 mole H
1.0 g H
= 1 7.23
= 37.23
Empirical Formula: BH3
What is the formula for a substance with a percent
composition of:
26.56% potassium, 35.41% chromium, and the remainder
oxygen?
100 g- 26.56 g - 35.41 g = 38.03 g O
26.56 g K x 1 mole K = 0.679 moles K 39.1 g K
35.41 g Cr x 1 mole Cr = 0.68 mole Cr 52.00 g Cr
38.03 g 0 x 1 mole O = 2.37 mole O 16.00 g O
= 1 0.679
= 10.679 = 3.490.679
Many salts absorb water out of the air to form a salt hydrate. You have 10 g of MgSO4 hydrate. After heating, it weighs 4.89 g. Why? How many water molecules are trapped inside a MgSO4 crystal?
MgSO4 ∙ ? H2O