Questions: Elemental phosphorus occurs as tetratomic molecules, P4. What mass of chlorine gas is needed to react completely with 495. g of phosphorus to form phosphorus pentachloride? Be sure your answer has the correct number of significant figures. P4 + 10Cl2 -> 4PCl5

Solution Steps

First, calculate the molar mass of elemental phosphorus (\(P_4\)) and chlorine gas (\(Cl_2\)).

• The atomic mass of phosphorus (\(P\)) is approximately 30.97 g/mol. Therefore, the molar mass of \(P_4\) is: \[ 4 \times 30.97 \, \text{g/mol} = 123.88 \, \text{g/mol} \]

• The atomic mass of chlorine (\(Cl\)) is approximately 35.45 g/mol. Therefore, the molar mass of \(Cl_2\) is: \[ 2 \times 35.45 \, \text{g/mol} = 70.90 \, \text{g/mol} \]

Next, calculate the number of moles of \(P_4\) in 495 g of phosphorus.

\[ \text{Moles of } P_4 = \frac{495 \, \text{g}}{123.88 \, \text{g/mol}} \approx 3.995 \, \text{mol} \]

According to the balanced chemical equation: \[ P_4 + 10Cl_2 \rightarrow 4PCl_5 \]

1 mole of \(P_4\) reacts with 10 moles of \(Cl_2\). Therefore, 3.995 moles of \(P_4\) will react with:

\[ 3.995 \, \text{mol} \times 10 = 39.95 \, \text{mol of } Cl_2 \]

Finally, calculate the mass of \(Cl_2\) needed using its molar mass.

\[ \text{Mass of } Cl_2 = 39.95 \, \text{mol} \times 70.90 \, \text{g/mol} \approx 2832 \, \text{g} \]

Final Answer