# Homework Help: Trouble finding where I went wrong calculating concentration

Tags:
1. Aug 10, 2017

### Trufflehog

In our laboratory class we were required to calculate the stock (mM) and final (µM) concentrations for Glycerol. The tutor said my final calculations were incorrect and that I had made an error. He didn't have enough time show me where I went wrong. I have compared my results with another student who was told their calculations were correct.

I have expanded my calculations to try and identify the problem but I cannot see where I am going wrong. Is there an error in my calculations?

2. Aug 10, 2017

### mjc123

Your calculations look OK to me, except the very last step. 1M is not 109µmol. What is it?
I wonder if the "25" and "540" should have been mL, not µL. Certainly, 3.65 µL in 25 µL (or 3.65 mL in 25 mL, or L in L..) is quite a high concentration; of the order of 2M sounds the right ballpark, even without doing any calculations. I can't see where the number 73.6 comes from.

3. Aug 10, 2017

### Trufflehog

Thanks for noticing the error where I should of had 10^6 µM. Yeah we definitely used µL for all those solutions in the lab. I had discussed it with the tutor and all he said was that my answers did not match his. He did however tell me that another student got the correct answers and I was able to copy those down. I thought maybe it's possible that my tutor got the incorrect result but if I were to challenge that I thought I'd check with someone else calculations first.

Thanks mjc123