Question

Consider the laboratory on HPLC analysis of components in diet soft drinks. Your standard solutions for caffeine were prepared from a stock solution of 0.40 mg/mL caffeine, from which you drew, 1.0, 2.0, 3.0, 4.0 and 5.0 mL and diluted each one to 50.0 mL. Calculate the concentration of the most concentrated standard solution.

For the same lab as in problem 1, suppose you plotted peak height (y-axis) vs. concentration in mg/mL (x-axis), and the slope was found to be 25.0 mL/mg. If the peak height for the unknown was 15.5, calculate the concentration in the unknown. Assume the intercept was zero.

Show working solution, thanks!

Answer 1

Mass = Concentration of stock solution * Volume added

= 0.4 mg/mL * 1 mL = 0.4 mg

[Standard], mg/mL = Mass / Total Volume

= 0.4 mg / 50 mL = 0.008 mg/L

Similarly for other standards, the concentration can be obtained as shown below:

[Stock], mg/mL	Vol added (mL)	Mass, mg	Total Volume (mL)	[Standard], mg/mL
0.4	1	0.4	50	0.008
0.4	2	0.8	50	0.016
0.4	3	1.2	50	0.024
0.4	4	1.6	50	0.032
0.4	5	2	50	0.04

a)

Concentration of most concentrated standard solution = 0.04 mg/mL

b)

y = m x

Peak height = y

Unknown concentration, mg/mL = x

y = 25 x

y = 15.5

x = 15.5 / 25

= 0.62 mg/mL