Topic 3 mathematics of formulas and equations
I.
Finding gram formula mass (gfm)/ formulas mass:
use periodic table
-is the sum of the atomic masses of all the atoms
present
Gram formula mass, gram (g) is the unit
Formula mass, atomic mass unit (amu) is the unit
Ex 1: NaCl
Ex 2: 2H2O
Mass of Na = 23
Mass of H = 1x4
=4
Mass of Cl = 35
Mass of O = 16 x 2
= 32
Total mass = 23 + 35 = 58g
Total mass = 4 + 32 =36g
Problems:
1) What is the gram formula mass of Ca(NO3)2?
2) Which substance has the greatest molecular mass?
a) H2O2
b) NO
c) I2
d)CF4
II. Percentage Composition – a substance represents the
composition as a percentage of each element compared with the total
mass of the compound.
Ex 1 : What is the percentage of oxygen in H2O?
Percent Composition
1) Look up formula in Table T
2) Find the part and the whole
% = mass of part/ mass of Whole x 100
mass of part = O = 16g
mass of whole = H2O=18g
For H: 2 atoms x 1 =2
For O: 1 atom x 16 =16
Total mass = 2+16=18g
3) Put numbers into the formula
% = 16 / 18 x 100 = 89% Oxygen
Ex 2: What is the percentage, by mass of water in sodium carbonate
crystals, Na2CO3∙
10H2O?
1) Look up formula in Table T
2) Find the part and the whole
% = mass of part/ mass of whole x 100
mass of part = 10H2O
mass of whole = Na2CO3∙ 10H2O
For H: 20 atoms x 1 =20amu
For O: 10atoms x 16 = 160amu
Total mass = 180amu
For Na: 2 atoms x 23
For C: 1 atom x 12
For O: 3 atoms x 16
For H2O: 10 units x 18
= 46amu
= 12amu
= 48amu
=180amu
Total mass = 46 + 12 + 48 + 180=286amu
3) Put numbers into the formula
% = 180 / 286 x 100 = 63% water
III.
Mole- is defined as the number of atoms of carbon
present in 12.000grams of C-12. Chemists use a specific
collective noun to define a particularly usable number of
particles.
Ex 1: How many grams are present in 40.5mol of sulfuric acid (H2SO4)?
Steps:
1) Look up formula in Table T
number of mole = given mass / gfm
2) Identify information
number of mole = 40.5mol
given mass = X
gfm (H2SO4)? =
For H:
2 atoms x 1
For S:
1 atom x 32
For O:
4 atoms x 16
= 2g
= 32g
= 64g
Total mass = 2 + 32 + 64 = 98g
3) Put numbers into the formula
4) Solve for X
40.5mol = X/98
X=3970g
Grams H2SO4 = 3970g
Ex 2: How many mole are equivalent to 4.75g of sodium hydroxide
(NaOH)?
Steps:
1) Look up formula in Table T number of mole = given mass / gfm
2) Identify information
number of mole = X
given mass = 4.75g
gfm (NaOH)? =
For Na:
1 atoms x 23
For O:
1 atom x 16
For H:
1 atoms x 1
= 23g
= 16g
= 1g
Total mass = 23 + 16 + 1 = 40g
3) Put numbers into the formula
4) Solve for X
X = 4.75/40
X=0.119mol
Moles NaOH = 0.119 mol
Question 1) What is the equivalent to 78g of LiF?
Question 2) What is the total mass of 1.5mol of SO2?
IV.
Find Molecular formulas from empirical formulas
Ex 1. A compound has a molecular mass of 180amu and an empirical
formula of CH2O. What is its molecular formula?
Identify the known and unknown
Molecular mass = 180amu
Empirical mass =
Molecular formula =?
Empirical formula = CH2O
1) From the empirical formula, we can find the empirical mass
CH2O
C = 1x12= 12
H = 2x1 = 2
O = 1x16= 16
Total = 30amu
2) Divide the molecular mass by the empirical mass.
180amu/ 30amu = 6
3) Multiply the subscripts in the empirical formula by 6.
C1x6 H2x6 O1x6 = C6 H12 O6
Ex 2. A compound has an empirical formula of CH2 and a molecular
mass of 28amu. What is its molecular formula?
Identify the known and unknown
Molecular mass = 28amu
Empirical mass =
Molecular formula =?
Empirical formula = CH2
1) From the empirical formula, we can find the empirical mass
CH2
C = 1x12= 12
H = 2x1= 2
Total = 14amu
2) Divide the molecular mass by the empirical mass.
28amu/ 14amu = 2
3) Multiply the subscripts in the empirical formula by 2.
C1x2 H2x2 = C2 H4
V.
Mole relations in balanced equations
*** Mole ratio***
Ex 1.
2C2H6 + 7O2 → 4CO2 + 6H2O
2
: 7
: 4
: 6
If there’s 4 C2H6, how many moles of H2O?
If there’s 1 CO2, how many moles of H2O?
If there’s 21 O2, how many moles of H2O?