Question

Concentrated hydrochloric acid is a 38% solution of HCL in water and has a density of 1.18g/ml. How many milliliters of concentrated hydrochloric acid are needed to prepare 500ml of 1:200 HCL solution? The answer is not 2.12ml
My calculation:
1.18g/ml x 1000ml x 0.38/36.5 = 12.28M
500ml x 1:200 = 2.5
12.28/2.5 = 4.9ml

Who helps me to solve it for me. Tell me the right answer. Please help

Answer 1

Sometimes biochemists (I assume you are in a bio class) use funny symbols and nomenclature (at least in my opinion). Here is a web site that will explain some about dilutions.
http://www.stolaf.edu/people/muth/Appendix%20II.pdf

If I read this correctly, 1:200 means you want 1 mL stock solution diluted to 200 mL total volume or 0.005. So if you want 500 mL instead of 200 mL, that will be 2.5 mL diluted to 500 total volume. From the way your problem is stated I don't know if that is 1:200 of the 12.28 M stuff OR if it means 1:200 of something else. My best guess is that it means you take 2.5 mL of the 12.28 M concd HCl and dilute to 500 mL.
All of that means the density, %HCl etc etc are just extraneous data. Of course I may have interpreted the problem incorrectly.