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

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

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