Question

a student weighs out a 21.1 - g sample of aluminum and a 21.1 - g sample of copper. which sample contains more atoms? why?
watch the words in the left column to the appropriate blanks in the sentences on the right.
the samples are of the same mass, but the metal with a larger molar mass is
for this reason, the sample contains more moles of metal and the sample contains more atoms.

Explanation:

Step1: Calculate molar mass

Molar mass of aluminum ($Al$) is $M_{Al}=26.98\ g/mol$, molar mass of copper ($Cu$) is $M_{Cu} = 63.55\ g/mol$. So copper has a larger molar mass.

Step2: Calculate number of moles

Number of moles $n=\frac{m}{M}$. Given $m = 21.1\ g$ for both. For aluminum $n_{Al}=\frac{21.1}{26.98}\approx0.782\ mol$, for copper $n_{Cu}=\frac{21.1}{63.55}\approx0.332\ mol$. So aluminum sample has more moles.

Step3: Relate moles and atoms

Since $N = n\times N_{A}$ ($N_{A}$ is Avogadro's number), more moles means more atoms. So aluminum sample has more atoms.

Answer:

The samples are of the same mass, but the metal with a larger molar mass is copper. For this reason, the aluminum sample contains more moles of metal and the aluminum sample contains more atoms.