What you are given
- Reagent (optional): NaCl
- Molar mass (g/mol): 58.44
- Concentration: 0.1 M
- Volume: 100 mL
Formula: n = m / MW ; c = n / V
Concentration
How much solid do I weigh out for a given molarity?
mass = concentration x volume x molar mass. Keep concentration in mol/L and volume in L, and the answer comes out in grams. If the reagent is a hydrate, use the hydrate molar mass, not the anhydrous one.
Worked example
Every number below is computed by the calculator, not typed in
The working
- Derive amount:
n = c · V=0.01 - Derive mass:
m = n · M=0.5844
That is 0.5844 g of NaCl.
What each symbol means
- m: Mass to weigh (g)
- C: Concentration (mol/L)
- V: Volume (L)
- MW: Molar mass (g/mol)
A printable one-page version is on the formula sheet.
Where this usually goes wrong
- Using the anhydrous molar mass for a hydrated salt. CuSO4 is 159.6 g/mol, CuSO4 5H2O is 249.7 g/mol, a 56% error.
- Leaving volume in mL. 100 mL is 0.1 L, so a mL figure gives an answer 1000 times too large.
- Ignoring assay purity on the bottle. At 95% purity you need to weigh 1/0.95 as much.
Try it yourself
Two problems, with the working revealed
Problem 1 Molarity, mass and volume conversion
Work out the mass to weigh.
- Solve for: Molarity
- Molar mass (g/mol): 58.44
- Volume: 100 mL
- Mass: 0.3 g
Show the answer and the working
That is 0.05133 M.
- Derive amount:
n = m / M=0.005133 - Derive concentration:
c = n / V=0.05133
Open this in the calculator to change the numbers.
Problem 2 Molarity, mass and volume conversion
Work out the mass to weigh.
- Solve for: Volume
- Molar mass (g/mol): 58.44
- Concentration: 0.1 M
- Mass: 3.84 g
Show the answer and the working
That is 657.1 mL.
- Derive amount:
n = m / M=0.06571 - Derive volume:
V = n / c=0.6571
Open this in the calculator to change the numbers.
Do it with your own numbers.
The Molarity, mass and volume conversion calculator is free, shows every step, flags the hazards, and works offline once loaded. No account, no signup.
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