What you are given
- Solve for: Moles
- Molar mass (g/mol): 121.76
- Mass: 5 g
Formula: n = m / MW
Concentration
How do you calculate molar mass from a formula?
Add the atomic mass of every atom in the formula. Subscripts multiply the atom before them, and everything inside a bracket is multiplied by the number after the bracket.
Worked example
Every number below is computed by the calculator, not typed in
The working
- Derive amount:
n = m / M=0.04106
That is 0.04106 mol of Sb.
Where this usually goes wrong
- Missing a bracket multiplier, for example reading Ca(OH)2 as one OH instead of two.
- Forgetting the water of hydration after the centre dot in a formula like CuSO4 5H2O.
- Rounding atomic masses too early. Round only the final answer.
Try it yourself
Two problems, with the working revealed
Problem 1 Molar mass, grams and moles
Work out the molar mass.
- Solve for: Moles
- Molar mass (g/mol): 180.16
- Mass: 200 mg
Show the answer and the working
That is 0.00111 mol of C6H12O6.
- Derive amount:
n = m / M=0.00111
Open this in the calculator to change the numbers.
Problem 2 Molar mass, grams and moles
Work out the molar mass.
- Solve for: Moles
- Molar mass (g/mol): 121.76
- Mass: 8 g
Show the answer and the working
That is 0.0657 mol of Sb.
- Derive amount:
n = m / M=0.0657
Open this in the calculator to change the numbers.
Do it with your own numbers.
The Molar mass, grams and moles calculator is free, shows every step, flags the hazards, and works offline once loaded. No account, no signup.
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