hypothesis testing, stuck at finding standard deviation

#1
here's the actual question:

The calculated nitrogen content of pure benzanilide is 7.10%. Five repeat analyses of "representative" samples yielded values of 7.11%, 7.08%, 7.06%, 7.06%, and 7.04%. Using an ALPHA level of size 5%, can we conclude that the experimental mean differs from the expected value? Assume that the measured values are approximately normally distributed.

i know i have to find the value of t critical and t samples, but to find the t samples i have to know the standard deviation of the population, how do i do that?