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Molar massChemistryHigh schoolabout 1:09

What is the molar mass of H3PO4?: animated lesson

A narrated, step-by-step molar mass video lesson with captions. Final answer: M(H3PO4) = 97.994 g/mol.

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Video with a synthetic English voice and captions. Pick another language for subtitles in that language, or open the interactive version.

Video narrated by a synthetic voice (Piper TTS, en_US-ljspeech-high voice (LJ Speech dataset, public domain)); the steps come from our maths engine and the answer was checked independently. Download MP4 (0.5 MB) · Captions (WebVTT)

Steps on screen

1. The problem
Worked example
What is the molar mass of H3PO4?
2. Steps, part 1
1. $\text{H}: 3,\quad \text{P}: 1,\quad \text{O}: 4$
  Count the atoms of each element
2. $\text{H} = 1.008,\quad \text{P} = 30.974,\quad \text{O} = 15.999$
  Look up each atomic weight (g/mol)
3. $3 \times 1.008 = 3.024 \\ 1 \times 30.974 = 30.974 \\ 4 \times 15.999 = 63.996$
  Multiply each count by its atomic weight
4. $M = 3.024 + 30.974 + 63.996 = 97.994\ \text{g/mol}$
  Add the contributions
3. Steps, part 2
1. $\%\text{H} = \frac{3.024}{97.994} \times 100 \approx 3.09\% \\ \%\text{P} = \frac{30.974}{97.994} \times 100 \approx 31.61\% \\ \%\text{O} = \frac{63.996}{97.994} \times 100 \approx 65.31\%$
  Mass percent of each element
4. The answer
$M = 97.994\ \text{g/mol}$
3×1.008 + 1×30.974 + 4×15.999 = 97.994 g/mol
5. Check
- Percent composition adds to 100.00%.
- Elements: hydrogen ×3, phosphorus ×1, oxygen ×4.

Transcript

The problem. Here is the problem: What is the molar mass of H3PO4? Add the atomic weights of all 8 atoms in one formula unit of H3PO4.

Steps, part 1. Count the atoms of each element. Subscripts multiply everything inside the brackets before them, and a hydrate coefficient multiplies the whole water (or other) unit. Look up each atomic weight (g/mol). IUPAC standard atomic weights (abridged, conventional values). Multiply each count by its atomic weight. Add the contributions.

Steps, part 2. Mass percent of each element.

The answer. So the answer is M(H3PO4) equals 97.994 g/mol. The molar mass is the mass of one mole of formula units. It equals the sum of the atomic weights of every atom in the formula, and numerically matches the formula mass in daltons (u).

Check. Percent composition adds to 100.00 percent. Elements: hydrogen times 3, phosphorus times 1, oxygen times 4.

About molar mass

Add atomic masses to find the molar mass of a compound from its formula.

What is the molar mass of H3PO4?: Molar mass Lesson