Grad entry medicine - GAMSAT purple booklet questions chemistry help needed!

mystery1

Hi All,

I'm sitting the GAMSAT next week for Grad Entry Medicine.

I sat down today to sit Paper III of the Purple ACER practice booklet and I got stuck on one or two questions

Can anyone help with Questions 46 and 48 of the Purple Booklet?

Calcium Oxalate (CaC2O4) is a major component of kidney stones. Oxalic acid, being a diprotic acid,has two acid dissociation constants as shown in the figure below. A typical sample of kidney fluid has a pH of 8.2 and a concentration of calcium ions of 1.5 mM.

HOOC-COOH (oxalic acid) --> H+ and HOOC-COO- : Ka1 = 6.5 x 10-2 M
HOOC-COO- --> H+ and -OOC-COO- : Ka2 = 6.1 x 10-5 M
Ca2+ + -OOC-COO- -->Ksp = 3.0 x 10-9M

Q46 What is the minumum concentration of oxalate ion needed to form kidney stones in typical kidney fluid?
a) 1.4 x 10-3M
b) 2.0 x 10-3M
c) 1.4 x 10-6M
d) 2.0 x 10-6M


Q48 Active transport removes some of the calcium ions from the kidney fluid so that they can be used elsewhere in the body (e.g. to produce bone mass).
In a kidney that contains a kidney stone, removing some of the calcium ions from the kidney fluid will:-

a) increase the pH of the kidney fluid
b)decrease the pH of the kidney fluid
c)increase the solubility product of calcium oxalate in the kidnet fluid
d)decrease the concentration fo oxalate ions dissolved in the kidney fluid


Any help that can be offered would be greatly appreciated, I'm very freaked out over this exam! :eek::eek::eek:

mystery1

Ok, I've figured out for Q48 that if I remove Ca2+ ions that there will be more -OOC-COO- in solution which will push the equilibrium of the 2 previous equations to the left, which means less H+ ions and therefore an increase in pH, so the answer is "a"

So its just Q46 I need the help with! What is this Ksp carry on?

Chrisso

So the concentration of Ca2+ x concentration of oxalate ion = the solubility constant which is 2.0 x 10^-9.

[Ca2+][-OOC-COO-] = 3.0 x 10^-9

We know the concentration of the Ca2+ which is 1.5mM/1000 = 1.5x10^-3

Therefore divide the Ksp by the concentration of Ca2+ and you are left with 2.0 x 10^-6...

mystery1

Thank you very much for your help Chrisso, much appreciated

Chrisso

Any idea on Q 89 and 90 in purple book... Physics is a real weak point for me, trying to get my head round concepts. Using the formula given, p=Ft and the equation for kinetic energy, 1/2mv^2 I am getting wrong answers... Think I am not reading the graph too well...

Any thoughts...?

Cheers

mystery1

Hey,

For Question 89, I'm not sure if I am 100% correct in my theory, but here's how I got the answer......

The momentum of the object before the collision is 0.15mg (mass of object) x 300 m/s (velocity).

This gives an initial momentum of 4.5 kg m/s before impact.

After impact, the momentum of the object on rebound can be calculated using the P=F x t formula provided. So momentum p after the collision is the Force at 0.10 seconds (when rebound begins) which is 300N, x time to when the force reaches zero which is 0.04 seconds .

300N x 0.04s is 12 N s.

The units "kg m/s" is the same as "N s" as far as I know.....

So the momentum changes from 4.5 N s (before rebound) to 12 N s (after rebound), so the answer is

12 Ns - 4.5 Ns = 7.5 Ns (answer a)


For Question 90


The KE for the ball before collision is 1/2MV2 = 1/2 * 0.15kg * (30 m/s)2
SO this gives a KE before collision of 67.5 Joules

After collision, from Q 89 we know the momentum has increased by 7.5 Ns
momentum = mass x velocity, and the mass hasn't changed, so velocity = 7.5/0.15 = 50 m/s

KE after collision using the same 1/2mv2 formula = 1/2 * 0.15mg * (50 m/s)2, so this gives a KE after collision of 187.5 J

So the change in KE is 187.5 minus 67.5 = an increase 120 Joules (answer C)

I hope this helps a bit, best of luck next week!

Chrisso

Great! I was closer than I thought... Feel free to fire across any other questions you have trouble with, particularly if they are chem related ;-)

Thanks for your help

mystery1

I'll hold you to that!! Thanks a mil

I've found a forum that has worked through answers for the green booklet, this may be useful http://helpgenie.co.uk/viewforum.php?f=7

I only had up to Junior Cert Physics up to a few weeks ago, there's a youtube site called "Khan Academy" which gives online lectures on loads of physics topics, I've found them brilliant

Feel free to fire any questions at me either, I will be glad to help and it helps/motivates me to review topics too

Suaimhneach

Moved from Graduate to post-graduate.