Question

Lakshmi has a sample of ammonium nitrate (NH4NO3) that has a mass of 40.10 g. She knows that the molar mass of NH4NO3 is 80.0432 g/mol. To the correct number of significant digits, how many moles of NH4NO3 does the sample contain?
0.50
0.500979
0.500980
0.5010

Answer 1

The answer is 0.5010

Number of moles (n) is equal to the quotient of mass (m) and molar mass of a sample (Mr):
n = m/Mr

We have:
n = ?
m = 40.10 g
Mr = 80.0432 g/mol

n = 40.10 g : 80.0432 g/mol = 0.5010

40.10 has 4 significant digits,
80.0432 has 6 significant digits.
Since 4 is less than 6, we choose 4 significant digits