[kinerd]

Creatinine, C₄H₇N₃O, is a by-product of muscle metabolism, and creatinine levels in the body are known to be a fairly reliable indicator of kidney function. The normal level of creatinine in the blood for adults is approximately 1.0 mg per deciliter (dL) of blood.
a. If the density of blood is 1.025 g/mL, calculate the molality of a normal creatinine level in a 10.0-mL blood sample.
b.What is the osmotic pressure of this solution at 25.0^oC?

For (a) I tried using stoichiometry to get mol/kg, but it did not work. I started out with 1.0 mg/dL, converted that to grams, then to moles using the molar mass of creatinine, then to L, and finally to kg using 1025 kg/L (density). I don't know how to incorporate the 10.0-mL blood sample into this. Did I do something wrong?

Thanks.

[Borek]

Volume of the sample is meaningless.

[kinerd]

Why is the volume meaningless? Wouldn't molality change with volume?

[sjb]

Why is the volume meaningless? Wouldn't molality change with volume?

Assuming it's well mixed, then it doesn't matter whether the sample is 1 ml, 10 ml, 1 litre or 5000 litres, they will all have the same concentration.

[kinerd]

Ok, I understand now. Thank you.
I want to make sure that I'm doing the problem right, though. Are my calculations correct? My final answer is 8.62 x 10^-2 mol/kg. It seems a bit small, but considering that the normal level is 1.0 mg per deciliter (dL) of blood, it seems ok.

[Borek]

10^-2 mol/kg

Bit small? Looks way too high for me.

As for why volume doesn't matter... You have two samples of 0.1 mol/L hydrochloric acid. One is 10 mL, other is 100 mL. Which has a higher concentration?

[kinerd]

Sorry! I meant 8.62 x 10^-8 mol/kg. And yes, I understand your point Borek. They would both have the same concentration. Thanks!

[Borek]

Sorry! I meant 8.62 x 10^-8 mol/kg.

That seems too low.

And yes, I understand your point Borek. They would both have the same concentration. Thanks!

At least that's settled then