SME
General Chemistry I · Problem type
Water of hydration from a heating experiment
Use this for “Heating 3.00 g of a copper(II) sulfate hydrate leaves 1.92 g of CuSO₄. Find x in CuSO₄·xH₂O.”
Chart
This is an example. Type your own values to start a new problem.
mhyd
Hydrate mass
example · g
mₐₙₕ
Anhydrous mass
example · g
mw
Water mass
calculated · g
nw
Moles of water
calculated · mol
M
Salt molar mass
example · g/mol
nₛ
Moles of salt
calculated · mol
x
Water per formula unit
calculated
k
Water in the formula
calculated
Formulas
mw = mhyd − mₐₙₕ
1.08 = 3 − 1.92
nw
=
mw
18.02
0.05993
=
1.08
18.02
nₛ
=
mₐₙₕ
M
0.01203
=
1.92
159.61
x
=
nw
nₛ
4.982
=
0.05993
0.01203
k
=
x
rounded
to
the
ones
5 = 4.982 rounded to the ones
Assumptions
•
Heating drives off all the water and nothing else, so the mass lost is the water.
•
Water is 18.02 g/mol. The salt’s molar mass is the anhydrous formula’s: CuSO₄ is 159.61 g/mol.
•
x counts the water molecules for each formula unit of salt, so it is rounded to the nearest whole number: 4.98 is 5.
Step-by-step
WE KNOW
mhyd = 3 g, mₐₙₕ = 1.92 g, M = 159.61 g/mol
FIND
water mass (mw), moles of water (nw), moles of salt (nₛ), water per formula unit (x), water in the formula (k)
Step 1 · Find water mass (mw)
Use mw = mhyd − mₐₙₕ
The mass lost on heating is the water that left.
mw = 3 − 1.92
mw = 1.08 g
Step 2 · Find moles of water (nw)
Use nw = mw ÷ 18.02
Divide the grams of water by 18.02 g/mol.
nw
=
1.08
18.02
nw = 0.05993 mol
Step 3 · Find moles of salt (nₛ)
Use nₛ = mₐₙₕ ÷ M
Divide the dry salt’s grams by its molar mass.
nₛ
=
1.92
159.61
nₛ = 0.01203 mol
Step 4 · Find water per formula unit (x)
Use x = nw ÷ nₛ
Divide by the moles of salt to count the water for each formula unit.
x
=
0.05993
0.01203
x
=
4.982
Step 5 · Find water in the formula (k)
Use k = x rounded to the ones
A
formula
holds
whole
molecules
of
water:
write
the
salt,
a
dot,
then
k
H₂O.
k
=
4.982
rounded
to
the
ones
k
=
5
Check
Put the numbers back in. Both sides should match.
1.08 = 3 − 1.92 ✓
0.05993
=
1.08
18.02
✓
0.01203
=
1.92
159.61
✓
4.982
=
0.05993
0.01203
✓
5 = 4.982 rounded to the ones ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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