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Question

By usual analysis, $1.00\text{ g}$ of compound (X) gave $1.79\text{ g}$ of magnesium pyrophosphate. The percentage of phosphorus in compound (X) is : __________ (nearest integer)
(Given, molar mass in $\text{g\ mol}^{-1}$ : $\text{O} = 16, \text{Mg} = 24, \text{P} = 31$)

The correct answer is
$50$

Phosphorus Percentage Calculation in Compound X

This solution details the calculation for determining the percentage of phosphorus in compound (X) based on the gravimetric analysis yielding magnesium pyrophosphate ($\text{Mg}_2\text{P}_2\text{O}_7$).

1. Molar Mass Calculations

Calculate the molar mass of magnesium pyrophosphate ($\text{Mg}_2\text{P}_2\text{O}_7$) using the given atomic masses:

  • Molar mass of Mg = $24 \text{ g mol}^{-1}$
  • Molar mass of P = $31 \text{ g mol}^{-1}$
  • Molar mass of O = $16 \text{ g mol}^{-1}$

Molar mass of $\text{Mg}_2\text{P}_2\text{O}_7 = (2 \times \text{M}(\text{Mg})) + (2 \times \text{M}(\text{P})) + (7 \times \text{M}(\text{O}))$

Molar mass of $\text{Mg}_2\text{P}_2\text{O}_7 = (2 \times 24) + (2 \times 31) + (7 \times 16)$

Molar mass of $\text{Mg}_2\text{P}_2\text{O}_7 = 48 + 62 + 112 = 222 \text{ g mol}^{-1}$.

2. Mass of Phosphorus Determination

Determine the total mass of phosphorus (P) present in one mole of $\text{Mg}_2\text{P}_2\text{O}_7$.

Mass of P in $\text{Mg}_2\text{P}_2\text{O}_7 = 2 \times \text{M}(\text{P}) = 2 \times 31 = 62 \text{ g mol}^{-1}$.

3. Phosphorus Percentage Calculation

Calculate the actual mass of phosphorus derived from the $1.79 \text{ g}$ of magnesium pyrophosphate obtained:

Mass of P = Mass of $\text{Mg}_2\text{P}_2\text{O}_7 \times \frac{\text{Mass of P}}{\text{Molar mass of Mg}_2\text{P}_2\text{O}_7}$

Mass of P = $1.79 \text{ g} \times \frac{62 \text{ g mol}^{-1}}{222 \text{ g mol}^{-1}}$

Mass of P $\approx 0.50 \text{ g}$.

Finally, calculate the percentage of phosphorus in the original $1.00 \text{ g}$ sample of compound (X):

Percentage of P = $ \frac{\text{Mass of P}}{\text{Mass of Compound X}} \times 100\% $

Percentage of P = $ \frac{0.50 \text{ g}}{1.00 \text{ g}} \times 100\% $

Percentage of P = $50\%$.

Final Answer Conclusion

The calculated percentage of phosphorus in compound (X) is $50\%$. This matches the nearest integer value provided in the options.

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