1. MATH OF CHEMISTRY

2. Compounds are electrically neutral.
A compound is a substance composed of two or more different elements that are chemically combined in a fixed proportion. A chemical compound can be broken down by chemical means.
Compounds are electrically neutral.
J Deutsch 2003

3. The subscript 1 is never written in a formula
A chemical formula is both qualitative and quantitative. It tells which elements are in the compound with symbols and how many of each with subscripts.
The formula for sulfuric acid is H2SO4
Element Number of atoms H S O
The subscript 1 is never written in a formula
J Deutsch 2003

4. The formula mass must be a whole number multiple of the empirical mass
Molecular Empirical Multiple
Formula Mass Formula Mass
H2O HO
C3H CH
C6H12O CH2O
Hg2O HgO
J Deutsch 2003

5. The formula mass of a substance is the sum of the atomic masses of its atoms. The molar mass (gram formula mass) of a substance equals one mole of that substance. (3.3e)
Formula mass of H2SO4 H 2 x 1 = 2
S 1 x 32 = 32
O 4 x 16 = 64
98 amu
J Deutsch 2003

6. The formula mass represents the mass of one molecule of a substance while the gram formula mass represents the mass of a mole (6.02 x 1023 molecules) of that substance.
The calculation for formula mass and gram formula mass (GFM) are the same, the difference is in the units.
Formula Mass of H2SO4 = 98 amu
Gram Formula Mass of H2SO4 = 98 g
If they tell you to use the mass rounded to the nearest tenth then use tenths for each mass as well as your final answer.
J Deutsch 2003

7. In all chemical reactions there is a conservation of mass, energy, and charge.
Matter cannot be created nor destroyed, only changed from one form to another
Energy cannot be created nor destroyed, only changed from one form to another
J Deutsch 2003

8. A balanced chemical equation represents conservation of atoms.
A mole of molecules is made up of 6.02 x 1023 molecules
A chemical equation is balanced to ensure the conservation of matter/energy, and charge.
The total mass before the reaction must equal the total mass after the reaction has taken place.
J Deutsch 2003

9. Chemical equations must be balanced so that mass can be conserved.
Word equation: hydrogen + oxygen water
Chemical equation: 2H2 + O H2O
J Deutsch 2003

10. Regents Question: 08/02 #9
If an equation is balanced properly, both sides of the equation must have the same number of
(1) atoms
(2) coefficients
(3) molecules
(4) moles of molecules þ
2H2 + O H2O
J Deutsch 2003

11. How many on the left? How many on the right? What do I do?
Balance the reaction Fe + O Fe2O3
There are 2 oxygen atoms on the left and 3 on the right. To get them equal I need to multiply the left by 3 and the right by 2. These multiples are called coefficients and are placed in front of the formula. The yield sign and the plus separate the formulas.
Fe + 3O Fe2O3
Now the number of iron atoms has to be balanced. There is one iron on the left and four on the right. Use a coefficient of 4 in front of the Fe on the left.
4Fe + 3O Fe2O3
Now it is correctly balanced
J Deutsch 2003

12. Regents Question: 06/03 #59
Given the reaction between two different elements in the gaseous state
Box A below represents a mixture of the two reactants before the reaction occurs.
The product of this reaction is a gas. Draw the system after the reaction has gone to completion, based on the Law of Conservation of Matter.
One reactant is in excess.
Box A – System before reaction
Box B – System after reaction
J Deutsch 2003

13. Use the coefficients to predict the amount of reactant consumed or product formed (Mole-Mole problems)
The ratio of the coefficients is a ratio of the moles taking part in a reaction.
Reactants (on the left of the arrow) are consumed
Products (on the right of the arrow) are formed
Given the number of moles of any substance in a reaction, you can use the coefficients to find the number of moles of any other substance.
J Deutsch 2003

14. Place the moles given over the coefficient for that substance and set up a proportion with the coefficients.
How many moles of oxygen are produced when 4 moles of KClO3 react according to the equation: 2 KClO KCl + 3 O2 X
2 KClO KCl + 3 O2
4 = X
cross multiply to get 2X = 12
X=6 moles
unknown
given
J Deutsch 2003

15. Regents Question: 01/03 #42 þ Given the equation:
2C2H2(g) + 5O2(g) CO2(g) + 2H2O(g)
How many moles of oxygen are required to react completely with 1.0 mole of C2H2?
2.5
2.0
5.0 10 þ
J Deutsch 2003

16. Regents Question: 06/02 #37 þ Given the reaction:
6CO2 +6H2O C6H12O6 + 6O2
what is the total number of moles of water needed to make 2.5 moles of C6H12O6 ?
2.5
6.0 12 15 þ
J Deutsch 2003

17. Regents Question: 06/03 #20 þ Given the reaction:
PbCl2 (aq) + Na2CrO4 (aq) PbCrO4 (s) + 2NaCl(aq)
What is the total number of moles of NaCl formed when 2 moles of Na2CrO4 react completely?
(1) 1 mole
(2) 2 moles
(3) 3 moles
(4) 4 moles þ
J Deutsch 2003

18. Regents Question: 06/02 #34
A compound has a gram formula mass of 56 grams per mole. What is the molecular formula for this compound?
CH2
C2H4
C3H6
C4H8 þ
J Deutsch 2003

19. Regents Question: 06/02 #41 þ The gram formula mass of NH4Cl is
22.4 g/mole
28.0 g/mole
53.5 g/mole
95.5 g/mole
N 1 x 14.0 =
H 4 x =
Cl 1 x 35.5 = _______
g/mole þ
J Deutsch 2003

20. Grams/GFM = Moles To convert grams into moles, divide the grams by the gram formula mass To convert moles into grams, multiply by the moles by the gram formula mass The unit for gram formula mass is grams/mole
How many moles in 49 grams of H2SO4?
49 g = X moles X= 0.50 moles
98 g/mol
How many grams are contained in 2.00 moles of H2SO4?
X = moles X = 196 grams
J Deutsch 2003

21. The percent composition by mass of each element in a compound can be calculated mathematically. (3.3f)
Calculate the formula mass and then divide the component of the element you are looking for by the total mass of the formula then multiply by 100%.
Formula mass of H2SO4 % of Oxygen in H2SO4
H 2 x 1 = /98 x 100% = 65% S 1 x 32 = O 4 x 16 =
J Deutsch 2003

22. Regents Question: 01/03 #8
What is the percent by mass of oxygen in H2SO4? [ formula mass = 98]
16%
33%
65%
98%
64/98 x 100 þ
J Deutsch 2003

23. Regents Question: 06/02 #7
The percent by mass of hydrogen in NH3 is equal to
17 x 100 1
(2) 17 x 100 3
1 x 100 17
(4) x 100 þ
J Deutsch 2003

24. Regents Question: 06/03 #10
The percent by mass of calcium in the compound calcium sulfate (CaSO4 ) is approximately
15%
(2) 29%
(3) 34%
(4) 47% þ
J Deutsch 2003

25. A hydrate is a compound which has water trapped in its crystal structure. We can determine the percentage of water in a hydrate.
CuSO45H2O – for every copper(II) sulfate there are 5 trapped water molecules
Find the formula mass including the water
Cu 1 x 64 = 64
S 1 x 32 = 32
O 4 x 16 = 64
H x 1 = 10 Water contributes 90 to
O 5 x 16 = 80 the 250
/250 x 100% = 36%
J Deutsch 2003

26. Regents Question: 06/02 #36
What is the total number of oxygen atoms in the formula MgSO4•7H2O? [The • represents seven units of H2O attached to one unit of MgSO4] 11 7 5 4 þ
J Deutsch 2003