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StoichiometryChemistryHigh schoolabout 1:27

Grams of H2O from 4 g of H2: animated lesson

A narrated, step-by-step stoichiometry video lesson with captions. Final answer: 2H2 + O2 → 2H2O; 4 g H2 → H2O: 35.744 g (1.98413 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
In the reaction H2 + O2 → H2O, how many grams of H2O form from 4 g of H2?
2. Steps, part 1
1. $\text{H}: 2c_{1} = 2c_{3} \\ \text{O}: 2c_{2} = 1c_{3}$
  Write an atom balance for each element
2. $c_{1} = 2,\ c_{2} = 1,\ c_{3} = 2$
  Solve the linear system with the smallest whole numbers
3. $2\mathrm{H_{2}} + \mathrm{O_{2}} \rightarrow 2\mathrm{H_{2}O}$
  Balanced equation
4. $n = \frac{4\ \text{g}}{2.016\ \text{g/mol}} = 1.98413\ \text{mol}$
  Moles of H2
3. Steps, part 2
1. $n = 1.98413 \times \frac{2}{2} = 1.98413\ \text{mol},\quad m = 1.98413 \times 18.015 = 35.744\ \text{g}$
  H2O via the mole ratio 2 : 2
4. The answer
$2\mathrm{H_{2}} + \mathrm{O_{2}} \rightarrow 2\mathrm{H_{2}O}$
2H2 + O2 → 2H2O
5. Check
- H: 4 = 4, O: 2 = 2 ✓
- H2O is produced in a 2 : 2 mole ratio to H2.

Transcript

The problem. Here is the problem: In the reaction H2 plus O2 → H2O, how many grams of H2O form from 4 g of H2? Balance atoms of every element, then use the coefficients as mole ratios.

Steps, part 1. Write an atom balance for each element. Give each species an unknown coefficient; atoms of every element must be equal on both sides. Solve the linear system with the smallest whole numbers. Exact elimination leaves one free coefficient; scaling by the least common denominator gives integers. Balanced equation. Moles of H2.

Steps, part 2. H2O via the mole ratio 2: 2.

The answer. So the answer is 2H2 plus O2 → 2H2O; 4 g H2 → H2O: 35.744 g (1.98413 mol). Coefficients in a balanced equation count particles (moles), not grams. Convert grams to moles, apply the mole ratio, then convert back to grams with the other species prime molar mass. Assumes the reaction goes to completion with the known species fully consumed (or formed).

Check. H: 4 equals 4, O: 2 equals 2, which checks out. H2O is produced in a 2: 2 mole ratio to H2.

About stoichiometry

Use a balanced equation to find how many grams of product form from a given mass of reactant.

Grams of H2O from 4 g of H2: Animated Step-by-Step Lesson