Topic 3 mathematics of formulas and equations
- 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: NaClEx 2: 2H2O
Mass of Na = 23Mass of H = 1x4 =4
Mass of Cl = 35Mass of O = 16 x 2= 32
Total mass = 23 + 35 = 58gTotal 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) NOc) I2d)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% = mass of part/ mass of Whole x 100
2)Find the part and the wholemass 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% = mass of part/ mass of whole x 100
2)Find the part and the whole
mass of part = 10H2Omass of whole = Na2CO3∙ 10H2O
For H: 20 atoms x 1 =20amuFor Na: 2atoms x 23 = 46amu
For O: 10atoms x 16 = 160amuFor C: 1atomx 12 = 12amu
Total mass = 180amuFor O: 3 atoms x 16= 48amu
For H2O: 10 units x 18 =180amu
Total mass = 46 + 12 + 48 + 180=286amu
3)Put numbers into the formula% =180 / 286 x 100 = 63% water
- 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 = 2g
For S: 1 atom x 32= 32g
For O: 4 atoms x 16 = 64g
Total mass = 2 + 32 + 64 = 98g
3)Put numbers into the formula40.5mol = X/98
4)Solve for XX=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 Tnumber of mole = given mass / gfm
2)Identify informationnumber of mole = X
given mass = 4.75g
gfm (NaOH)? =
For Na: 1 atoms x 23 = 23g
For O:1 atom x 16= 16g
For H: 1 atoms x 1 = 1g
Total mass = 23 + 16 + 1 = 40g
3)Put numbers into the formulaX = 4.75/40
4)Solve for XX=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?
- Find Molecular formulas from empirical formulas
Ex1. 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 = 180amuMolecular formula =?
Empirical mass = Empirical formula = CH2O
1)From the empirical formula, we can find the empirical mass
CH2OC = 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 = 28amuMolecular formula =?
Empirical mass = Empirical formula = CH2
1)From the empirical formula, we can find the empirical mass
CH2C = 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
- Mole relations in balanced equations
*** Mole ratio***
Ex 1.2C2H6 + 7O2 → 4CO2 + 6H2O
2 : 7 : 4 : 6
If there’s 4C2H6,how many moles of H2O?
If there’s 1CO2, how many moles of H2O?
If there’s 21O2, how many moles of H2O?