Moles to Grams, Grams to Moles Conversion Worksheet Key ... - Free Printable

Let’s go step by step through each problem on the worksheet. I’ll check every calculation carefully before giving the final answer.

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Problem 1: How many moles are in 15 grams of lithium? (molar mass of lithium is 7 g/mole)

We use:
moles = grams ÷ molar mass

So:
15 g ÷ 7 g/mol = 2.142857... mol

Rounded to two decimal places → 2.14 moles

Wait — the student wrote “6.94 grams” as the molar mass? That’s wrong for this problem. The problem says “molar mass of lithium is 7 g/mole”, so we must use 7, not 6.94.

But let’s double-check: actual atomic mass of Li is ~6.94 g/mol, but since the problem explicitly says “use 7”, we follow instructions.

→ 15 ÷ 7 = 2.142857 ≈ 2.14 moles

However, the student wrote 2.16 — that would be if they used 6.94:
15 ÷ 6.94 ≈ 2.16 → which matches their work.

BUT — the problem says: *“molar mass of lithium is 7 g/mole”* → so we should use 7.

This is a conflict between real-world value and given value. Since it’s a worksheet with explicit instruction, we follow the given value: 7 g/mol.

So correct answer: 15 ÷ 7 = 2.14 moles

But wait — looking at the student’s work again: they crossed out “grams” and wrote “6.94 grams” under the conversion factor — meaning they ignored the problem’s instruction and used real molar mass.

Since the problem says “(molar mass of lithium is 7 g/mole)”, we MUST use 7.

✔ Final for #1: 2.14 moles

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Problem 2: How many grams are in 2.4 moles of sulfur? (molar mass of sulfur is 32 g/mole)

Use:
grams = moles × molar mass

2.4 mol × 32 g/mol = ?

2.4 × 32:

Break it down:
2 × 32 = 64
0.4 × 32 = 12.8
Total = 64 + 12.8 = 76.8 grams

Student wrote 76.97 — that’s because they used 32.07 (actual molar mass). But problem says “32 g/mole”.

So we use 32.

✔ Final for #2: 76.8 grams

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Problem 3: How many moles are in 22 grams of argon?

Molar mass of argon (Ar) = 39.95 g/mol (standard value; not given, so we use periodic table value)

Student used 39.95 — correct.

moles = 22.00 g ÷ 39.95 g/mol

Calculate:

22 ÷ 39.95 ≈ ?

Do division:
39.95 × 0.55 = 39.95 × 0.5 = 19.975; 39.95 × 0.05 = 1.9975 → total 21.9725 → very close to 22

So 22 ÷ 39.95 ≈ 0.5507 mol

Rounded to two decimals → 0.55 moles

Student got 0.55 — correct.

✔ Final for #3: 0.55 moles

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Problem 4: How many grams are in 88.1 moles of magnesium?

Molar mass of Mg = 24.31 g/mol (standard; student used this)

grams = moles × molar mass
= 88.1 × 24.31

Let’s compute:

First, 88 × 24.31 = ?

80 × 24.31 = 1944.8
8 × 24.31 = 194.48
→ 1944.8 + 194.48 = 2139.28

Now 0.1 × 24.31 = 2.431

Total = 2139.28 + 2.431 = 2141.711 grams

Student wrote 2141.71 — correct.

✔ Final for #4: 2141.71 grams

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Problem 5: How many moles are in 2.3 grams of phosphorus?

Molar mass of P = 30.97 g/mol (student used this)

moles = 2.3 ÷ 30.97

Calculate:

30.97 × 0.07 = 2.1679
30.97 × 0.074 = 30.97 × 0.07 = 2.1679; 30.97 × 0.004 = 0.12388 → total 2.29178 → close to 2.3

So 2.3 ÷ 30.97 ≈ 0.07426...

Rounded to two decimal places → 0.07 moles

Student wrote 0.07 — acceptable rounding.

But let’s be precise:
2.3 ÷ 30.97 = 0.074265... → rounds to 0.07 if two sig figs? Wait — 2.3 has two sig figs, 30.97 has four → limiting is two sig figs.

0.074 → rounded to two sig figs is 0.074? No — leading zeros don’t count. So 0.074 has two sig figs? Actually, 7 and 4 — yes, two sig figs after the first non-zero digit.

Standard rule: 0.074 has two significant figures (7 and 4).

But often in such problems, they expect two decimal places or match input precision.

Input: 2.3 g → one decimal place, two sig figs.

Answer: 0.074 mol → which is 7.4 × 10⁻² → two sig figs.

Student wrote 0.07 — which is one sig fig. That’s too rough.

Better to write 0.074 moles

Check calculation again:

2.3 ÷ 30.97 = ?

Do exact:
2.3 / 30.97 = 230 / 3097 ≈ ?

3097 × 0.074 = 3097 × 0.07 = 216.79; 3097 × 0.004 = 12.388 → total 229.178 → close to 230

Difference: 230 - 229.178 = 0.822 → so add 0.822 / 3097 ≈ 0.000265 → total ≈ 0.074265

So 0.0743 moles if rounded to three decimals, or 0.074 moles for three sig figs? Input 2.3 has two sig figs → answer should have two.

0.074 has two sig figs (the 7 and 4). Yes.

Writing 0.07 implies one sig fig — incorrect.

So better answer: 0.074 moles

But student wrote 0.07 — probably due to rounding early.

Given that, and since the problem doesn’t specify sig fig rules strictly, but in science we usually match least precise measurement.

2.3 has two sig figs → answer should have two.

0.074 → the '7' and '4' are significant → two sig figs → correct.

0.07 → only '7' is significant → one sig fig → wrong.

So we correct to 0.074 moles

✔ Final for #5: 0.074 moles

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Now, also note: the top part asks for molecular masses — let’s verify those quickly since they’re part of the worksheet.

1) NaOH:
Na = 22.99, O = 16.00, H = 1.01 → 22.99 + 16.00 + 1.01 = 40.00 g/mol → student correct.

2) H₃PO₄:
H: 1.01×3 = 3.03
P: 30.97
O: 16.00×4 = 64.00
Total: 3.03 + 30.97 + 64.00 = 98.00 g/mol → student correct.

3) H₂O:
H: 1.01×2 = 2.02
O: 16.00
Total: 18.02 g/mol → student correct.

4) Mn₂Se₇:
Mn: 54.94×2 = 109.88
Se: 78.96×7 = 552.72
Total: 109.88 + 552.72 = 662.60 g/mol → student correct.

5) MgCl₂:
Mg: 24.31
Cl: 35.45×2 = 70.90
Total: 24.31 + 70.90 = 95.21 g/mol → student wrote 95.11 — mistake!

Wait: 24.31 + 70.90 = 95.21, not 95.11.

Student did: 24.31 + (35.45)2 → 35.45×2=70.90, then 24.31+70.90=95.21

They wrote 95.11 — typo? Or misadded?

24.31 + 70.90:

24 + 70 = 94
0.31 + 0.90 = 1.21 → total 95.21

Yes — student error. Should be 95.21 g/mol

6) (NH₄)₂SO₄:
N: 14.01×2 = 28.02
H: 1.01×8 = 8.08 (since NH₄ has 4H, two NH₄ → 8H)
S: 32.07
O: 16.00×4 = 64.00
Total: 28.02 + 8.08 = 36.10; +32.07 = 68.17; +64.00 = 132.17 g/mol → student correct.

So correction needed for #5 molecular mass: 95.21 g/mol, not 95.11.

But the main "solve the following" section is what we focus on for final answers.

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Final Answers Summary:

1) 15 g Li → using given 7 g/mol → 15/7 = 2.142857 → 2.14 moles
2) 2.4 mol S → 2.4 × 32 = 76.8 grams
3) 22 g Ar → 22 / 39.95 = 0.5507 → 0.55 moles
4) 88.1 mol Mg → 88.1 × 24.31 = 2141.711 → 2141.71 grams
5) 2.3 g P → 2.3 / 30.97 = 0.074265 → 0.074 moles

Note: For #1 and #2, even though real molar masses differ, we follow problem instructions.

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Final Answer:
1) 2.14 moles
2) 76.8 grams
3) 0.55 moles
4) 2141.71 grams
5) 0.074 moles