Question

An old (pre-1987) penny is nearly pure copper. If such a penny has a mass of 3.3 g, how many moles of copper atoms would be on one penny?

Answer 1

Answer:

Approximately 3.1 × 10²².

Explanation:

Start by considering: how many moles of copper atoms in this 3.3-gram coin?

To answer that, it would be necessary to know the mass of one mole of copper atoms. The mass (in grams) of one mole of copper atoms is numerically equal to the relative atomic mass of copper. Look up the relative atomic mass of copper on a modern periodic table.

• Cu: 63.546.

In other words, the mass of one mole of copper atoms is equal to 63.546 grams. Hence, the number of moles of copper atoms in the 3.3-gram coin would be equal to

$\displaystyle n = \frac{m}{M} = \frac{3.3}{63.546} \approx 0.0519309 \; \rm mol$.

How many copper atoms would that be? Avogadro's Constant gives the number of particles in one mole.

$N_A \approx 6.023\times 10^{23}\; \rm mol^{-1}$.

Since there are approximately $0.0519309 \; \rm mol$ copper atoms, the number of copper atoms would be

$\displaystyle N = \frac{n}{N_A} \approx \frac{0.0519309}{6.023\times 10^{23}} \approx 3.3 \times 10^{22}$.