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  1. Problems
  2. /A drop of water has a mass of 0.072 g. How many molecules of water are in the drop? …
HardChemistryHigh School (9–10)GeneralAtoms and Molecules › The Mole ConceptNumericalBoard Exam

A drop of water has a mass of 0.072 g. How many molecules of water are in the drop? …

StemPy LabContributed by StemPy Lab

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

A drop of water has a mass of 0.072 g0.072\ \text{g}0.072 g. How many molecules of water are in the drop? How many atoms? Use NA=6.022×1023 mol−1N_A = 6.022 \times 10^{23}\ \text{mol}^{-1}NA​=6.022×1023 mol−1.

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Work it out on paper, then type your number. The answer itself stays hidden.

Need a reminder? The key ideaShowHide

One mole holds NA=6.022×1023N_A = 6.022 \times 10^{23}NA​=6.022×1023 particles. n=mMn = \dfrac{m}{M}n=Mm​ and N=n×NAN = n \times N_AN=n×NA​. To count atoms, multiply the molecules by the atoms in one molecule. The mass of one particle is MNA\dfrac{M}{N_A}NA​M​.

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

Step 1. Moles. 0.072 g18 g/mol=0.004 mol\dfrac{0.072\ \text{g}}{18\ \text{g/mol}} = 0.004\ \text{mol}18 g/mol0.072 g​=0.004 mol.

Step 2. Molecules. 0.004×6.022×1023=2.41×10210.004 \times 6.022 \times 10^{23} = 2.41 \times 10^{21}0.004×6.022×1023=2.41×1021.

Step 3. Atoms. 3×2.41×1021=7.23×10213 \times 2.41 \times 10^{21} = 7.23 \times 10^{21}3×2.41×1021=7.23×1021.

Answer: 2.41×10212.41 \times 10^{21}2.41×1021 molecules and 7.23×10217.23 \times 10^{21}7.23×1021 atoms.

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  1. Mole calculationsMolecular Mass and the Mole

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