SME
Analytical Chemistry · Problem type
Water hardness by EDTA titration
Use this for “A 75 mL tap-water sample takes 13.99 mL of 0.01000 M EDTA at pH 10. Report the hardness in mg/L as CaCO₃.”
Diagram
Molarity is moles per liter: 0.124 mmol in 12.4 mL is 0.01 mol/L.
Each dot is 5 × 10⁻⁶ mol of EDTA: 25 dots, about.
This is an example. Type your own values to start a new problem.
hardness (mg/L)
=
C (M)
×
V (mL)
×
100.09
g/mol
÷
Vₛ (mL)
Vₛ is the water sample’s volume; C and V are the EDTA’s concentration and the volume of it used.
n
Moles of metal ion
calculated · mmol
m
Mass as CaCO₃
calculated · mg
Formulas
n
=
C
×
V
0.124 = 0.01 × 12.4
m
=
n
×
100.09
12.41 = 0.124 × 100.09
hardness
=
m
Vₛ
×
1,000
248.2
=
12.41
50
×
1,000
Assumptions
•
EDTA binds each Ca²⁺ or Mg²⁺ ion one to one, so the moles of EDTA at the end point are the moles of metal ion.
•
Hardness is reported as if every metal ion were CaCO₃ (100.09 g/mol), in mg per liter of sample.
•
The titration runs at pH 10 with an indicator such as Eriochrome Black T.
Step-by-step
WE KNOW
Vₛ = 50 mL, C = 0.01 M, V = 12.4 mL
FIND
moles of metal ion (n), mass as CaCO₃ (m), hardness as CaCO₃ (hardness)
Step 1 · Find moles of metal ion (n)
Use n = C × V
Molarity times milliliters gives millimoles of EDTA, one per metal ion.
n
=
0.01
×
12.4
n
=
0.124
mmol
Step 2 · Find mass as CaCO₃ (m)
Use m = n × 100.09
Count every metal ion as CaCO₃: 100.09 mg per millimole.
m
=
0.124
×
100.09
m
=
12.41
mg
Step 3 · Find hardness as CaCO₃ (hardness)
Use hardness = m ÷ Vₛ × 1,000
Milligrams per milliliter of sample, times 1000 mL per liter, gives mg/L.
hardness
=
12.41
50
×
1,000
hardness = 0.2482 × 1,000
hardness = 248.2 mg/L
Check
Put the numbers back in. Both sides should match.
0.124 = 0.01 × 12.4 ✓
12.41 = 0.124 × 100.09 ✓
248.2
=
12.41
50
×
1,000
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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