Stoichiometry: Grams to Grams Conversion - Step-by-Step Guide ... - Free Printable

Let’s solve this step by step.

We are given the balanced chemical equation:

2 H₂ + O₂ → 2 H₂O

This means:
- 2 molecules of hydrogen gas react with 1 molecule of oxygen gas to make 2 molecules of water.
- In terms of moles: 2 moles H₂ : 1 mole O₂ : 2 moles H₂O

We’re also told that we start with 3.0 grams of H₂, and we want to find out how many grams of H₂O we can make — assuming there’s plenty of O₂ (so H₂ is the limiting reactant).

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Step 1: Convert grams of H₂ to moles of H₂

Molar mass of H₂ = 2 × 1.0 g/mol = 2.0 g/mol

So,

> Moles of H₂ = mass / molar mass = 3.0 g / 2.0 g/mol = 1.5 moles H₂

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Step 2: Use the mole ratio from the balanced equation to find moles of H₂O produced

From the equation:
2 moles H₂ → 2 moles H₂O

That’s a 1:1 ratio between H₂ and H₂O!

So, if we have 1.5 moles of H₂, we’ll get 1.5 moles of H₂O.

*(Because 2 mol H₂ makes 2 mol H₂O → so 1 mol H₂ makes 1 mol H₂O)*

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Step 3: Convert moles of H₂O to grams of H₂O

Molar mass of H₂O = (2 × 1.0) + 16.0 = 18.0 g/mol

So,

> Mass of H₂O = moles × molar mass = 1.5 mol × 18.0 g/mol = 27.0 grams

Wait — but looking at the image, it says “Answer: 27.0 grams of H₂O” for the first example. But then in the second example, starting with 4.0g H₂, answer is 36.0g H₂O? Let me check that too.

Actually, let’s double-check our math.

If 2.0 g H₂ → 18.0 g H₂O (since 2g H₂ is 1 mole, which gives 1 mole H₂O = 18g)

Then 3.0 g H₂ should give:

(3.0 / 2.0) × 18.0 = 1.5 × 18.0 = 27.0 g H₂O ✔

Now, what about the next one? Starting with 4.0 g H₂:

Moles H₂ = 4.0 / 2.0 = 2.0 moles → produces 2.0 moles H₂O → 2.0 × 18.0 = 36.0 g H₂O ✔

Next: 5.0 g H₂ → 5.0 / 2.0 = 2.5 moles → 2.5 × 18.0 = 45.0 g H₂O ✔

Last one: 8.0 g H₂ → 8.0 / 2.0 = 4.0 moles → 4.0 × 18.0 = 72.0 g H₂O ✔

All match the answers shown in the image.

But wait — the user didn’t ask a specific question. They just uploaded an image titled “Intro to Stoichiometry – Grams to Grams NOTES”.

Since no specific problem was asked, I assume they want us to verify or explain the method used in the examples.

Looking again — actually, the last line says:

> “How many grams of H₂O are made when 8.0 grams of H₂ reacts?”

And the answer shown is 72.0 grams of H₂O

Let’s confirm that one carefully.

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Final Check: 8.0 grams H₂ → ? grams H₂O

Step 1: Moles of H₂ = 8.0 g ÷ 2.0 g/mol = 4.0 moles H₂

Step 2: Mole ratio H₂ to H₂O is 2:2 → same as 1:1 → so 4.0 moles H₂ → 4.0 moles H₂O

Step 3: Mass of H₂O = 4.0 mol × 18.0 g/mol = 72.0 grams

✔ Correct.

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Final Answer:
72.0 grams of H₂O