Acid rain, caused by the reaction of certain air pollutants with rainwater, is
a growing problem in the United States. Pure rain falling through clean air
registers a pH value of 5.7 (pH is a measure of acidity: 0 is acid; 14 is
alkaline). Suppose water samples from 40 rainfalls are analyzed for
\(\mathrm{pH},\) and \(\bar{x}\) and \(s\) are equal to 3.7 and \(.5,\) respectively.
Find a \(99 \%\) confidence interval for the mean \(\mathrm{pH}\) in rainfall and
interpret the interval. What assumption must be made for the confidence
interval to be valid?