mastering stoichiometry problems

STOICHIOMETRY MADE SIMPLE


Atomic mass of elements = Mass in amu
Molar mass = Total mass of elements in an atom or molecule (g) /mole
Example:
g/6.023 x1023 amu x 12 amu/1atom C x 6.023 x1023 atoms/mole = 12 g /mole Carbon
For solving stoichiometry problems, be familiar with the various types of problems and use
the correct unit factors and dimensional analysis, including names and units for all
substances in equation.
In general, start with the given value and use appropriate unit conversion factors to cancel
out unwanted units, multiply or divide the numbers and derive the answer.


1. Calculating Moles and Number of atoms for an element:
Problem: How many Fe atoms and how many moles of Fe atoms are present in 500 g of
iron?

500 gFe 1mole
X
MolarMass( g ) Fe

500 gFe 1mole
= X = 8.95 moles Fe.
55.85 gFe

6.022 X 1023 atoms
8.95 moles Fe X
1mole

= 5.39 X 1024 atomsFe


2. Calculating Molar Mass (g/mol) of a compound:
Problem: Calculate the (formula) molar masses of N2H4 and Ca3(PO4)2
(Add the molar masses of each element in a compound to obtain molar mass of
compound)
Molar Mass of N2H4 = 32 g/mole
Molar Mass of Ca3(PO4)2 = 310.3 g/mole


3. Calculating number of molecules in a compound:
Problem: How many molecules of NH3 are present in 2g of the substance?

, 2 gNH 3 1mole 6.022 X 1023 molecules
X X
MolarMass( g ) NH 3 1mole


2 gNH 3 1mole 6.022 X 1023 molecules
= X X
17 gNH 3 1mole


= 7.08 X 1022 moleculesNH 3


4. Calculating mass of a sample of atoms:
Problem: Calculate the mass in grams of a sample of americium containing six atoms.

6atomsAm AtomicMass(amu ) g
X X
1atomAm 6.022 X 1023 amu



6atomsAm 243amu g
= X X
1atomAm 6.022 X 1023 amu

= 2.42 X 10−21 g

5. Calculating mass in grams of a molecule from moles:
Problem 1: What is the mass (in g) of 1.5 X102 moles N2?

1.5 X 102 molesN 2 MolarMass( g ) N 2
X
1moleN 2

1.5 X 102 molesN 2 28 gN 2
= X
1moleN 2

= 4200 gN 2
= 4.2 X 103 gN 2