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What's the Ksp of MgCO3 when .0000019 g dissolve in 250 ml of water at 25 degrees C

Submitted by shortstummper5 on Sat, 08/09/2008 - 22:29

please help me figure out these two problems! i don't even know where to begin! please show steps and explain!

1) What's the Ksp of MgCO3 when .0000019 g dissolve in 250 ml of water at 25 degrees C?

2) The Ksp for PbI2 is 7.9 x 10^-9.  What are the products and their concentration?


1) Ksp= [ Mg2+][CO32-] where these are molar concentrations.  So convert the mass 1.9 X 10-6 to moles then convert to molarity for  the 250 mL solution.  Notice that the concentration of the magnesium ion and the carbonate ion will be the same.

2) Notice if s represents the concentration of the soluable ions then Pb2+ = s but I- = 2s so then Ksp = [Pb2+][I-]^2  where the iodine is squared. So

Ksp = [S][2S]^2
Ksp = S x 4S^2 = 4S^3
7.9 X 10^-9 = 4S^3  so divide by 4 and then take the cube root for S. The convert to the concentrations for Pb2+ and I-.

agaubr