[cloud5]

This is from my book. The bold part is the part that I need explaination.
Why 1/4 moles of BaSO⁴? and why Number of moles of potash alum in 1 dm³ solution
= 1.29 x 10^-3?


An alum tablet weighing 2.20g is dissolved in distilled water and made up 1dm³ solution. When 25.0cm³ of 0.10 mol dm^-3 barium chloride is added to 25.0cm³ of the solution, 0.03g of white precipitate is formed.


Q: Determine the % by mass of potash alum in the tablet.


A: Ba²⁺ + SO^2-₄ --> BaSO₄

Number of moles of BaSO₄ = 1.29 x 10^-4

Number of moles of potash alum in 25.0cm³ solution
= 1/4 moles of BaSO⁴
= 1/4 x 1.29 x 10^-4
= 3.23 x 10^-5

Number of moles of potash alum in 1 dm³ solution
= 1.29 x 10^-3

Mass of potash alum = 1.29 x 10^-3 x 948 = 1.22g

% by mass of potash alum = 1.22/2.20 x 100 = 55.5%

[Borek]

Seems to me like both 1/4 and 948 g/mol are wrong, but lets check the details.

What is potash alum formula?

[cloud5]

Seems to me like both 1/4 and 948 g/mol are wrong, but lets check the details.

What is potash alum formula?

Potash alum formula: K₂SO₄.Al₂(SO₄)₃.24H₂O

[Borek]

OK, I usually write it (and see it written) as a KAl(SO₄)₂·12H₂O. Your notation changes the situation.

How many moles of SO₄^2- groups per 1 mole of alum?

Can you write reaction equation for alum (in your version) reacting with BaCl₂?

[cloud5]

OK, I usually write it (and see it written) as a KAl(SO₄)₂·12H₂O. Your notation changes the situation.

How many moles of SO₄^2- groups per 1 mole of alum?

Can you write reaction equation for alum (in your version) reacting with BaCl₂?

I'm not sure this is the correct equation:
K₂SO₄.Al₂(SO₄)₃.24H₂O + BaCl₂ -->4BaSO₄ + K₂Cl₃ + Al₂Cl₅ + 24H₂O


I'm confused in this formula:
Number of moles of potash alum in 1 dm³ solution
= 1.29 x 10^-3

Why isn't the answer suppose to be 1.29 x 10^-7 because it is 1dm³ (divide 1000)?