Solving stoichiometry problems

Stoichiometry problems commonly involve two substances which appear in the equation for a chemical reaction. In these you are asked to find the mass (or concentration or volume) of one substance using information given about the second substance.

Below is a summary of the approach that BestChoice uses to solve stoichiometry problems.

Identify the Known substance.

Enough information is given to calculate the amount in moles (n) of the Known.
The amount in moles can be calculated
EITHER from the mass (m) of a pure substance by dividing by its molar mass (M)
OR from the concentration (c) and volume (V) of a solution.

Identify the Unknown substance.

Assign symbols to the given data and to the unknown quantity.

Be sure to include the substance to which each symbol refers, for example c(NaOH).
This helps you to connect the data to any mathematical relationships that you have learned.

Connect the Known and Unknown by identifying any intermediate quantities to be calculated.

Stoichiometry problems usually involve relating amounts in moles (n) of two substances.

Consider the example where substance A and B react according to aA + bB

cC + dD
The concentration and volume of A are given as well as the volume of B. The concentration of B is to be calculated.

Known substance A	Intermediate values					Unknown substance B
c(A) and V(A) concentration and volume of A	step 1	n(A) moles of A	step 2	n(B) moles of B	V(B) step 3	c(B) concentration of B

Note that some given data [like V(B)] may not be used until late in the calculation.
A mass can also be calculated in Step 3 using the molar mass of B and the relationship shown at the right.

Identify the mathematical relationships required to do each step of the calculation.

The three relationships (or rearranged forms of these) shown on this page are the most common for stoichiometry calculations.

Determine the calculated quantity (unknown).

This is the result of doing the calculation with no early rounding, that is, carrying through all digits.

Determine the reported quantity

The reported quantity is the calculated quantity given to a number of significant figures consistent with the given data. Thus the number of significant figures in this answer is the same as the lowest number of significant figures in given data used in the calculation.

n =	m
	M

n = cV

used in
step 1

n(A)	=	n(B)
a	=	b

used in
step 2

c =	n
	V

m = Mn

used in
Step 3