Download Mole Unit Study Guide key

Mole Unit Study Guide
ANSWER KEY
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Part I: Concepts and Understanding
1. How many items are there in 1 mole?
1 mole = 6.02 x 1023 items
2. What is the molar mass?
the mass of 1 mole of an element or a compound
similar terms: molar mass, molecular mass, molecular weight, formula mass, formula weight
3. How many oxygen atoms in the compound:
a. Fe2(SO4)3
12
b. (NH4)2CO3
3
c. Co(CH3COO)2
4
d. Ca3(PO4)2·5H2O
13
4. What equality do you need to convert an amount in grams to moles? Where could you get that equality?
molar mass.
For elements, get it from the periodic table. For compounds, add the mass of each element in the formula.
5. What equality do you need to convert an amount in L to moles? What conditions must be given?
Molar volume: 1 mole = 22.4 L . This equality can only be used for gases at standard temperature and pressure (STP)
6. What is % composition?
Percent by mass of each element in the compound
7. How do you calculate % composition?
% composition of an element = (total mass of element / molar mass of compound)*100%
8. What is an Empirical Formula?
A chemical formula that gives only the simplest whole number ratio of atoms of each element in a
compound.
9. What is a molecular formula? What is the difference between Empirical and. Molecular Formula?
A chemical formula that tells exactly how many atoms of each element in a single molecule of a compound
Molecular formula is a multiple of Empirical formula. Empirical formula could represent more than 1
compounds with the same ratio. Molecular formula is unique for 1 compound. Empirical formula gives ratio.
Molecular formula gives exact number of atoms
10. What are hydrates? How do you calculate molar mass of hydrate? How do you calculate % of water in
hydrate?
Hydrates are crystals of ionic compound that contains water molecules.
% of water in hydrate = (mass of water / molar mass of compound)*100%
Part II: Calculation
1. Molar mass:
a. Ca(NO3)2
1 Ca = 1 x 40.0
2 N = 2 x 14.0
6 O = 6 x 16.0
Molar mass =
40
28
96
164 g/mol
b. (NH4)2HPO4
132 g/mol
c. CuNO3 •5 H2O 216 g/mol
2. Conversions
a. How many moles are there in 5.0 g of Calcium Nitrate?
Ca(NO3)2
5.0 g
Molar mass = 164 g/mol
1mol
164 g
= 0.30 mol
b. What is the mass in grams of 2.34 moles of Calcium Nitrate?
2.34 mol
164 g
1 mol
= 384 g
c. How many moles are there in 5.0 L of Carbon Dioxide at STP?
5.0 L
1 mol
22.4 L
= 0.20 mol
d. What is the volume in L of 2.34 moles of Carbon Dioxide at STP?
2.34 mol
22.4 L
1 mol
= 52.4 L
e. What is the mass of 6.78 L of Carbon Dioxide at STP? (2 steps) CO2 molar mass = 44.0 g/mol
6.78 L
f.
1 mol
22.4 L
44.0 g
1 mol
= 13.3 g
What I the volume of 89.10 g of Carbon Dioxide at STP? (2 steps)
89.10 g
1 mol
44.0 g
22.4 L
1 mol
=45.4 L
3. Hydrates
Calculate the percent of water in CuNO3 •5 H2O
Molar mass = 216 g/mol
5 H2O = 5 x 18.0 = 90.0 g
% water = 90.0/216* 100% =41.7%
4. Percent Compositions
Calculate the percent composition in (NH4)2HPO4
Molar mass = 132 g/mol
N: 14.0 x2 = 28.0
28/132 x100% =21.2%
H: 1.01 x9
6.89%
P: 31
23.5%
O: 16.0 x 4 = 64.0
48.5%
5. Empirical Formula: A compound contains 63.0% C, 8.8 % H, and 28.2 % O. Find its empirical formula.
Element
Mass (g)
Molar mass
Moles
C
H
O
63.0
8.8
28.2
12.0
1.01
16.0
5.25
8.72
1.76
Empirical Formula = C3H5O
Ratio (divide by
small)
2.98
4.95
1
Final Ratio
3
5
1
6. Molecular Formula: A compound containing 63.0% C, 8.8 % H, and 28.2 % O has a molecular mass of 171
g/mole.
What is its molecular formula?
From # 5, the Empirical Formula = C3H5O
Empirical Mass = 57 g/mol
Given molar mass = 171 g/mol
171 /57 = 3
Molecular Formula = C9H15O3 (multiply subscripts of EF by 3 to get MF)
7. A household chemical containing 93.8% C and 6.2% H has a molecular mass of 128 g/mole. Find the molecular
formula for this compound. (Hint: Empirical formula first) What is the name of this compound? (Google its
formula)
Element
Mass (g)
Molar mass
Moles
C
H
93.8
6.2
12.0
1.01
7.81
6.14
So It is like 1.25 : 1
if you multiply by 4 you’ll get 5 : 4
Use the molar mass, you’ll get
C10H8 is Naphthalene or Mothballs
Ratio (divide by
small)
1.27
1
Final Ratio
5
4