Question

A sample of a solid labeled as NaCl may be impure. A student analyzes the sample and determines that it contains 75 percent chlorine by mass. Pure NaCl(s) contains 61 percent chlorine by mass. Which of the following statements is consistent with the data? (I know the answer is d but can someone plz explain to me why? Tyy)

a) The sample contains only NaCl(s).
b) The sample contains NaCl(s) and NaI(s).
c) The sample contains NaCl(s) and KCl(s).
d) The sample contains NaCl(s) and LiCl(s).

Answer 1

Answer: The sample contains NaCl(s) and LiCl(s).

Explanation:

1 mole of NaCl contains 1 mole of sodium atom and 1 mole of chlorine atom

1 mole of LiCl contains 1 mole of lithium atom and 1 mole of chlorine atom

To calculate the percentage composition of chlorine in sample, we use the equation:

$\%\text{ composition of chlorine}=\frac{\text{Mass of chlorine in sample}}{\text{Mass of sample}}\times 100$

• In NaCl compound:

Mass of NaCl = 58.5 g/mol

Mass of Chlorine atom = 35.5 g/mol

Putting values in above equation, we get:

$\%\text{ composition of chlorine}=\frac{35.5}{58.5}\times 100=61\%$

• In LiCl compound:

Mass of LiCl = 42.5 g/mol

Mass of Chlorine atom = 35.5 g/mol

Putting values in above equation, we get:

$\%\text{ composition of chlorine}=\frac{35.5}{42.5}\times 100=83.5\%$

Taking the average of mass percent of chlorine in both the compounds:

$\text{Percent mass of chlorine}=\frac{\text{Mass percent of chlorine in NaCl}+\text{Mass percent of chlorine in LiCl}}{2}$

Putting values in above equation, we get:

$\text{Percent mass of chlorine}=\frac{61+83.5}{2}\\\\\text{Percent mass of chlorine}=75\%$

Hence, the sample contains NaCl(s) and LiCl(s).