Summary of method

The amounts in moles of two substances in a chemical reaction are mathematically related through their coefficients in the balanced equation for the reaction. Thus if the mass of one of the substances is known, using the three-step process shown, the mass of the other substance can be calculated.

n =	m
	M

Calculate the amount in moles of known substance.

The amount in moles can be calculated from a given mass if the formula of the substance is known so that the molar mass can be calculated.

aA + bB

cC + dD
A, B, C, D are substances.
a, b, c, d are coefficients

n(A)	=	n(B)	=	n(C)	=	n(D)
a	=	b	=	c	=	d

Relate n(known) to

n(unknown)

through the coefficients of the balanced equation. Use any pair of the equations shown.

Hint for problem-solving: Set the equation up with the amount in moles of the known at the left. Then to find the amount of unknown, first divide the known amount by the coefficient of the known, then multiply by the coefficient of the unknown.

m = n × M

Calculate

m(unknown)

This can be viewed as a pathway between

m(known)

and m(unknown) if both the known and unknown are involved in the same chemical reaction.

Known

n =	m
	M

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

m(A)

n(A)

mass of A

Step 1

amount of A

Step 2

n(B) amount of B	m = n × M Step 3	Unknown m(B) mass of B

n(B)
amount of B

m = n × M

Step 3

Unknown
m(B)
mass of B

WARNING! All calculations in chemistry (and in BestChoice) should be done with attention to good mathematical practice

Use all digits in given data in your calculations
Do not round until the final answer, and then round up if the next digit is 5 or greater.
Pay attention to significant figures.

Digits after the first non-zero digit in a number are significant.
The number of significant figures in the answer should be equal to the lowest number of sigifnicant figures in the given data.