What Is the Mass of 3.81 mol of PH3? Solved Step by Step

If you have 3.81 moles of phosphine gas (PH₃), the mass is roughly 129.5 grams. The exact value depends on the atomic masses you use, but the calculation is straightforward: multiply the number of moles by the molar mass of the compound.

How to Calculate the Mass

First, find the molar mass of PH₃. Phosphorus (P) has an atomic mass of about 30.97 g/mol, and hydrogen (H) is about 1.008 g/mol. Since there are three hydrogen atoms, the total is:

• Phosphorus: 30.97 g/mol
• Hydrogen: 3 × 1.008 = 3.024 g/mol
• Molar mass of PH₃ = 30.97 + 3.024 = 33.994 g/mol

You can verify these atomic masses from standard reference data, such as the NIST Chemistry WebBook NIST.

Now apply the formula:

Mass = moles × molar mass
Mass = 3.81 mol × 33.994 g/mol = 129.5 g (rounded to three significant figures).

If you use slightly different atomic masses (e.g., 31.0 g/mol for phosphorus and 1.01 for hydrogen), the answer comes out to about 129.6 g. The difference is small and usually negligible for classroom problems.

For a quick check, try another example: what is the mass of 2.00 mol of PH₃? Multiply 2.00 × 33.994 = 67.99 g. The same method works for any mole quantity.

Key Takeaways

• The molar mass of PH₃ is 33.994 g/mol (using P = 30.97 and H = 1.008).
• To find the mass, multiply the number of moles by the molar mass: 3.81 mol × 33.994 g/mol = 129.5 g.
• Always check your periodic table for the atomic masses your instructor expects, as rounding can change the final digit.
• This calculation applies to any substance: find the molar mass first, then multiply by moles.