SS1 Chem concentration quest.

1. 4.00 g of sodium hydroxide is dissolved in 500 cm³ of water to make a solution. What is the concentration of the solution?
( Relative atomic mass: H = 1.0, O = 16.0, Na = 23.0)

A. 8.00 g dm³
B. 0.200 M
C. slightly higher than 0.200 M
D. slightly lower than 0.200 M

Ans: D

Explanation:
No. of moles of NaOH = 4.00/(23 + 16 + 1) = 1.00 mol
The volume of solution is greater than the volume of water used.
Hence, volume of the solution = (0.500 + ΔV) dm³
Concentration = [1.00/(0.500 + ΔV)] M < (1.00/0.500) M
Hence, concentration < 0.200 M


2. Which of the following solutions contain the same amount of ions?

(1) 36 cm³ of 1.0 M sodium nitrate solution
(2) 30 cm³ of 1.5 M sodium hydroxide solution
(3) 24 cm³ of 1 M potassium sulphate solution

A. (1) (2)
B. (1) (3)
C. (2) (3)
D. (1) (2) (3)

Ans: B

Explantion:
In (1):
No. of moles of NaNO₃ = 1.0 x (36/1000) = 0.036 mol
1 mole of NaNO₃ contains 2 moles of ions (1 mole of Na⁺ and 1 mole of NO₃⁻).
No. of ions = 0.036 x 2 = 0.072 mol

In (2):
No. of moles of NaOH = 1.5 x (30/1000) = 0.045 mol
1 mole of NaOH contains 2 moles of ions (1 mole of Na⁺ and 1 mole of OH⁻).
No. of ions = 0.045 x 2 = 0.090 mol

In (3):
No. of moles of K₂SO₄ = 1.0 x (24/1000) = 0.024 mol
1 mole of K₂SO₄ contains 3 moles of ions (1 mole of K⁺ and 2 moles of SO₄²⁻).
No. of ions = 0.024 x 3 = 0.072 mol

2010-06-06 00:39:51 補充：
Sorry. In (3), it should be 2 moles of K⁺ and 1 mole of SO₄²⁻.