Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: ConfusedBiologistHere on June 30, 2021, 05:07:03 PM

Firstly, sorry if this is the wrong form, I can move it if so, I tried my best.
Long story short, I was preparing 30% TCA solution as follows: 30g TCA into 100mL H2O.
However, my supervisor is telling me that this only equates to about a 14% solution?
His reasoning was that, a different protocol calls for 30 mL of 6.1N TCA to be combined with 70 mL of H2O. He explained that 6.1N TCA is made using 2.2g TCA into 1 mL of H20, which equals 100% (w/v). I do not understand this math at all. How could 2.2/1 = 100% and not 220%?
I'm probably missing something basic here and making a fool of myself, but I would really appreciate learning from someone! Thank you!

It is about as lousy as possible, and then a bit more :(
30 g of TCA in 100 mL doesn't produce 100 mL of the solution.
30 g of TCA in 100 mL produces solution that has a w/w concentration that reasonably accurately equals 30g/(100g+30g)*100% or 23%.
Density table for TCA solutions tells us that 23% w/w solution has a density of 1.12 g/mL. That allows us to calculate the volume: 116 mL.
That in turn means 30g in 116 mL, or w/v% around 26%.
The 14% number definitely doesn't come from from the basic definitions used for concentrations in chemistry. I feel like your supervisor assumes saturated solution to be 100%  they can define it this way, but it will be a highly unorthodox approach, at least from the chemical point of view.

I see.. so then it does make sense that a 6.1N TCA solution would be "100%" TCA? That is the information I get from my boss, but also the manufacturer. That is the other part I am confused about.
From what I have learned so far, we need to take into consideration the volume of TCA and how it effects the ending volume of the solution. That is why 30g in 100mL does not equal a 100mL solution of 30% TCA, but instead 116 mL solution of about 26% TCA. I'm definitely out of practice with my chem math, thank you so much for your help.

I see.. so then it does make sense that a 6.1N TCA solution would be "100%" TCA? That is the information I get from my boss, but also the manufacturer. That is the other part I am confused about.
I stand corrected by a quick googling, looks like 100% TCA is used quite correctly in the meaning "1g of TCA per 1 mL of solution", and yes, that's 6.1N. Nothing to do with the saturation (although this solution is definitely not far from being saturated).
I am not sure if I know how to reproduce the 14% number.
I was preparing 30% TCA solution as follows: 30g TCA into 100mL H2O.
Do you mean 30g of solid TCA, or 30 mL of the above 1 g/1mL solution?