A couple math questions

[vcuchic]

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hey guys,

so sorry for these massive amount of questions...
DATs are coming up soon, and yeh sort of freaking out here 🙁

1) Concentrated sulfuric acid is 18M. What volume of water would be needed to dilute 25 mL of concentrated H2SO4 solution to 3 M?

Answer: 125 mL

I keep getting 150 mL for this, by using the equation MV=MV

2) How many moles of AGIO3 (Ksp=3.1x10^-8) will dissolve in one liter of a 10^-5 M solution of NAIO3?

Answer: (sqrt 3.1x10^-8) - (10^-5)

I don't get why we would minus them instead of multiplying them together..is there an equation for Ksp?


Thank you!!

 

[phungy]

1) Use M1V1=M2V2

-> (18M)(0.025L)=(0.025L+x)(3M)
-> 0.45 = 0.075 + 3x
-> 0.375 = 3x
-> x = 0.125L = 125mL

 

[Lonely Sol]

2) How many moles of AGIO3 (Ksp=3.1x10^-8) will dissolve in one liter of a 10^-5 M solution of NAIO3?

Answer: (sqrt 3.1x10^-8) - (10^-5)

I don't get why we would minus them instead of multiplying them together..is there an equation for Ksp?


Thank you!!

This one is done on the basis of common ion effect:

First, realize that AgIO3 will diassociate into Ag+ and IO3-
so, ksp = 3.1x10-8 = x * x + 1x10-5<------------The reason for the addition is because we have to account for the extra iodate ion added to the reaction.

Now, it should end up with:

3.1x10-8 = x^2 + 1x10-5
3.1x10-8 - 1x10-5 = x^2
sqrt (3.1x10-8 - 1x10-5) = x

 

[vcuchic]

thank you guys so much!
really do appreciate it!