The expression “what is the mass of 3.81 mol of ph3” is a mole-to-mass conversion for phosphine, PH3. The mass follows \(m=n\times M\), where \(n\) is the amount in moles and \(M\) is the molar mass in \(\text{g·mol}^{-1}\).
Chemical identity of PH3
Phosphine, PH3, contains 1 phosphorus atom and 3 hydrogen atoms per molecule. Its molar mass is the sum of the atomic-mass contributions from P and H.
Molar mass (formula mass) of PH3
Using common periodic-table atomic masses \(M(\text{P})\approx 30.974\,\text{g·mol}^{-1}\) and \(M(\text{H})\approx 1.008\,\text{g·mol}^{-1}\), the molar mass is:
| Element | Count in PH3 | Atomic mass (\(\text{g·mol}^{-1}\)) | Contribution (\(\text{g·mol}^{-1}\)) |
|---|---|---|---|
| P | 1 | 30.974 | \(1\times 30.974=30.974\) |
| H | 3 | 1.008 | \(3\times 1.008=3.024\) |
| Total | 4 atoms | \(33.998\approx 34.00\) |
Mass from 3.81 mol of PH3
With \(n=3.81\,\text{mol}\) and \(M(\text{PH}_3)\approx 33.998\,\text{g·mol}^{-1}\), the mass is:
Reported value (significant figures)
\(3.81\) has 3 significant figures, so \(m\) rounds to \(1.30\times 10^{2}\,\text{g}\), which is \(130\,\text{g}\). Using \(M(\text{PH}_3)\approx 34.00\,\text{g·mol}^{-1}\) gives the same 3-significant-figure result.
Common pitfalls
- Unit consistency: grams require \(\text{g·mol}^{-1}\) for molar mass so that \(\text{mol}\) cancels.
- Subscript meaning: PH3 contains three hydrogen atoms, so the hydrogen contribution is \(3\times M(\text{H})\).
- Rounding practice: intermediate rounding that is too aggressive can drift the final value; carrying extra digits until the final report preserves accuracy.