Question

We want to estimate the true pH of a test tube of solution. The pH meter is known to have a standard deviation of 15. The margin of error for the pH of the solution must be no larger than 0.05, with 99% confidence. How many times should the pH be measured?

(a) 8

(b) 24

(c)25

(d)7

(e)59

(f)60

Answer 1

Solution :

Given that,

standard deviation =s =   =15=0.15 ph meter

Margin of error = E = 0.05

At 99% confidence level the z is,

= 1 - 99%

= 1 - 0.99 = 0.01

/2 = 0.005

Z/2 = 2.58

sample size = n = [Z/2* / E] 2

n = ( 2.58* 0.15 / 0.05)2

n =59.9

Sample size = n =60