SME
Principles of Biology I
Chemistry of life
Use this for “How many grams of NaCl (58.44 g/mol) make 500 mL of 100 mM saline?”
Diagram
Molarity is moles per liter: 0.05 mol in 500 mL is 0.1 mol/L.
Each dot is 0.002 mol of NaCl: 25 dots.
Mass of solute m = 2.922 g
Molar mass M = 58.44 g/mol
Molarity C = 100 mM
This is an example. Type your own values to start a new problem.
n (mol)
=
m (g)
÷
M (g/mol)
C (mM)
=
n (mol)
÷
V (mL)
Formulas
n
=
m
M
0.05
=
2.922
58.44
C
=
n
V
0.1
=
0.05
0.5
Formulas are worked in L, M. The step-by-step shows the conversions.
Assumptions
•
The solute dissolves completely; its molar mass M is the grams in one mole (NaCl: 58.44 g/mol).
•
V is the final volume of solution, not the volume of water added.
•
Molarity C is moles of solute per liter of solution: 1 M = 1 mol/L = 1000 mM.
Step-by-step
WE KNOW
m = 2.922 g, M = 58.44 g/mol, V = 500 mL
FIND
moles of solute (n), molarity (C)
Step 1 · Convert to the formula’s units
The formulas don’t work directly in the units you chose, so convert first (to L, M).
V = 500 mL = 0.5 L (1 L = 1000 mL)
Step 2 · Find moles of solute (n)
Use n = m ÷ M
Divide the grams by the grams in one mole.
n
=
2.922
58.44
n
=
0.05
mol
Step 3 · Find molarity (C)
Use C = n ÷ V
Molarity is the moles of solute in each liter of solution.
C
=
0.05
0.5
C
=
0.1
M
Step 4 · Convert the answers
Change each answer back to the unit you chose.
C = 0.1 M = 100 mM (1 M = 1000 mM)
Check
Put the numbers back in. Both sides should match.
0.05
=
2.922
58.44
✓
0.1
=
0.05
0.5
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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