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Q1.
A student investigated the temperature change in the reaction between dilute
sulfuric acid and potassium hydroxide solution.
This is the method used.
1. Measure 25.0 cm3 potassium hydroxide solution into a polystyrene cup.
2. Record the temperature of the solution.
3. Add 2.0 cm3 dilute sulfuric acid.
4. Stir the solution.
5. Record the temperature of the solution.
6. Repeat steps 3 to 5 until a total of 20.0 cm3 dilute sulfuric acid has been
added.
The student repeated the investigation.
The student used solutions that had different concentrations from the first
investigation.
The student found that 15.5 cm3 of 0.500 mol/dm3 dilute sulfuric acid
completely reacted with 25.0 cm3 of potassium hydroxide solution.
The equation for the reaction is:
2 KOH + H 2SO4 → K2SO4 + 2 H2O
Calculate the concentration of the potassium hydroxide solution in mol/dm3
and in g/dm3
Relative atomic masses (Ar): H = 1 O = 16 K = 39
Concentration in mol/dm3 = _____________________ mol/dm3
Concentration in g/dm3 = ______________________ g/dm3
(6)
-
Step 1: Calculate moles of sulfuric acid (H₂SO₄)
Moles of H₂SO₄
=
0.500
×
15.5
1000
=
0.00775
mol
Moles of H₂SO₄=
1000
0.500×15.5
=0.00775 mol
Step 2: Use ratio from equation (H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O)Moles of KOH
=
2
×
moles of H₂SO₄
=
2
×
0.00775
=
0.0155
mol
Moles of KOH=2×moles of H₂SO₄=2×0.00775=0.0155 mol
Step 3: Calculate concentration of KOH (mol/dm³)Conc. of KOH
=
moles
×
1000
volume
=
0.0155
×
1000
25.0
=
0.62
mol/dm³
Conc. of KOH=
volume
moles×1000
=
25.0
0.0155×1000
=0.62 mol/dm³
Step 4: Find Mr (Relative formula mass) of KOHMr of KOH
=
56
Mr of KOH=56
Step 5: Calculate concentration in g/dm³Conc. in g/dm³
=
Mr
×
Conc. in mol/dm³
=
56
×
0.62
=
34.7
g/dm³
Conc. in g/dm³=Mr×Conc. in mol/dm³=56×0.62=34.7 g/dm³
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