Question

A standard solution of ammonia is 100.0 ppm. You use an ion selective electrode (ISE) to measure ammonia of this standard solution and obtain the following data (in ppm): 99.4, 101.1, 100.4, 98.0, 99.6. What is the standard deviation of the mean of ammonia (in ppm) determined by the ISE? Report your answer to two significant figures.

I got 1.2 and that's incorrect.

Answer 1

Solution :

n = 5

= = 498.5 / 5 = 99.7

s =  (x - )² / n - 1  

= (99.4 - 99.7)² + (101.1 - 99.7)² + (100.4 - 99.7)²​​​​​​​ + (98 - 99.7)²​​​​​​​ + (99.6 - 99.7)²​​​​​​​ / 5 - 1

= 5.44 / 4

= 1.17