SME
Grade 10 Science · Problem type
Reading a solubility curve
Use this for “50 g of KNO₃ is stirred into 100 g of water at 40 °C. Is it saturated?”
Diagram
At 40 °C, 63.9 g of KNO₃ dissolves in 100 g of water.
50 g is unsaturated: 13.9 g more can dissolve.
Room left to dissolve: 13.9 g
This is an example. Type your own values to start a new problem.
T
Water temperature
example · °C
m
Salt stirred into 100 g of water
example · g
S
Most that dissolves in 100 g of water
worked out (not typed) · g
r
Room left to dissolve
calculated · g
Formulas
S
=
solubility
of
KNO₃
at
T
°C
63.9 = solubility of KNO₃ at 40 °C
r
=
S
−
m
13.9 = 63.9 − 50
Assumptions
•
The curve gives the most salt that dissolves in 100 g of water at each temperature.
•
Below the curve the solution is unsaturated; on it, saturated; past it (r below 0), the extra settles out.
Step-by-step
WE KNOW
T = 40 °C, m = 50 g
FIND
most that dissolves in 100 g of water (S), room left to dissolve (r)
Step 1 · Find most that dissolves in 100 g of water (S)
Use S = solubility of KNO₃ at T °C
Go up from the temperature to the curve, then across to the grams.
S
=
solubility
of
KNO₃
at
40
°C
S
=
63.9
g
Step 2 · Find room left to dissolve (r)
Use r = S − m
Take what is stirred in from what can dissolve; below zero, that much settles out.
r
=
63.9
−
50
r
=
13.9
g
is
above
0,
so
the
solution
is
unsaturated
r
=
13.9
g
Check
Put the numbers back in. Both sides should match.
63.9 = solubility of KNO₃ at 40 °C ✓
13.9 = 63.9 − 50 ✓
Topic check
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