SME
Grade 10 Science · Problem type
Molarity from grams of solute
Use this for “5.844 g of NaCl (58.44 g/mol) makes 250 mL of solution. What is the molarity?”
Diagram
Molarity is moles per liter: 0.1 mol in 250 mL is 0.4 mol/L.
Each dot is 0.005 mol of NaCl: 20 dots.
Mass of solute: 5.844 g
Molar mass of the solute: 58.44 g/mol
This is an example. Type your own values to start a new problem.
m
Mass of solute
example · g
Mₘ
Molar mass of the solute
example · g/mol
n
Moles of solute
calculated · mol
V
Volume of solution
example · mL
M
Molarity
calculated · mol/L
Formulas
n
=
m
/Mₘ
0.1 = 5.844/58.44
M
=
n
/(
V
1,000
)
0.4
=
0.1/(
250
1,000
)
Assumptions
•
Molarity is moles of solute per liter of solution, not of water: 1 M = 1 mol/L.
•
Change mL to L first: 1 L = 1000 mL.
•
Molar mass is the grams in one mole: NaCl is 58.44 g/mol.
Step-by-step
WE KNOW
m = 5.844 g, Mₘ = 58.44 g/mol, V = 250 mL
FIND
moles of solute (n), molarity (M)
Step 1 · Find moles of solute (n)
Use n = m/Mₘ
Divide the grams by the grams in one mole.
n
=
5.844/58.44
n
=
0.1
mol
Step 2 · Find molarity (M)
Use M = n/(V ÷ 1,000)
Change the milliliters to liters, then divide the moles by the liters.
M
=
0.1/(
250
1,000
)
M
=
0.1/0.25
M
=
0.4
mol/L
Check
Put the numbers back in. Both sides should match.
0.1 = 5.844/58.44 ✓
0.4
=
0.1/(
250
1,000
)
✓
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