Calculate pH-dependent solubility equilibrium?

Hello everyone, I've been given this task. My approach would be to calculate Ks, total, and K1 to determine a total dissociation constant. However, I don't quite understand how to include the 5g of sediment (converted to concentration), since this also affects the concentration of S 2- in the solution, and I can't simply subtract…

Which whey protein do you recommend? I'd like to order one that's easily soluble?

Because the previous products that I ordered (those were from the brand Mammut and once this house brand from Amazon, I think it was called Amfit) were both not very soluble and the powder always settled in the bottom of the shaker and stuck