Question

You have two aqueous solutions: solution A is 20 mL of 1 M MgSo₄. Solution B a 25 mL of 1.5 M Na₂SO₄.

You take 5 mL of solution A and mix it with 10 mL of solution B.

a. What are the total moles of ions in solution A?

b. What are the total moles of ions in solution B?

c. What are the total moles of ions in solution C?

d. What are the total moles of sulfur atoms in solution A?

e. What are the total moles of sulfur atoms in solution B?

f. What are the total moles of sulfur atoms in solution C?

g. Rate the relative electrical conductivity of these solutions (1=least conductive, 3=most)

Answer 1

a. Number of moles of MgSO4 in 20 mL of 1M soultion = Molarity x volume (litre)

= 1 x 0.020 = 0.020 moles

Since, MgSO4 is made up of two ions, Mg⁺⁺ and SO4^--

So, total number of moles of ions: 2 x moles of MgSO4 = 2 x 0.020 = 0.040 moles

b. Similarly number of moles of Na2SO4 in 25 mL of 1.5 M solution: 1.5 x 0.025 = 0.0375 moles

Since one mole of Na2SO4 gives three tions (2 Na⁺ and 1 SO4^--)

So total moles of ions in B will be = 3 x 0.0375 = 0.1125 moles

c. 20 ml of A has 0.040 mole of ions

   so, 5 mL of A will have 5 x 0.040/20 moles = 0.01 moles

Similarly 10 mL of solution B will have 10 x 0.0375/25 = 0.015 mole

So, total number of moles = 0.01 + 0.015 = 0.025 moles

d. one mole of MgSO4 has one mole of Sulfur

So, number of moles of sulfur in solution A = number of mole of MgSO4 = 0.020

e. One mole of Na2SO4 contains one mole of sulfur

   Number of moles of sulfur in Solution A = number of moles of Na2SO4 = 0.0375 moles

f. 20 mL of MgSO4 (solution A) 0.020 mole of sulfur

so, 5 mL will have: 5 x 0.02/20 = 0.005 mole of sulfur

25 mL of solution B has 0.0375 mole of sulfur

so, 10 mL of this will have: 10 x 0.0375/25 = 0.015 mole of sulfur

Total moles of sulfur in solution C = 0.01 + 0.015 = 0.025 mole

Concentration of ions in Solution A = number of moles/volume (lit) = 0.040/0.020 = 2M

Concentration of ions in Solution B = 0.1125/0.025 = 4.5 M

Concentration of ions in Solution C = 0.025/0.015 = 1.67 M

g. Conductance is proportional to the concentration of ions

So, the order for the conductance will be:

A (2)

B (3)

C (1)