CHE 315 – Lecture 5 9/9/05 Statistics

Quiz 2 1.An iron ore was analyzed by dissolving a g sample in concentrated HCl. After precipitation, filtration and washing, the residue was ignited to give g of pure Fe 2 O 3 ( g/mol). What was the % Fe ( g/mol) in the iron ore? 2.Stormwater running off of parking lots have typical copper (63.55 g/mol) concentrations of 100 ppb. Convert that concentration to molarity.

Arithmetic mean (average) Best way to express results if data is symmetrical

Standard Deviation Measures how closely the data is clustered around the mean Degrees of freedom: n-1

Variance = s 2 Relative standard deviation –(coefficient of variance)

Gaussian distribution Approximate µ by and σ by s

z Used to express deviations from the mean in multiples of the standard deviation

Probabilities Range Percentage of measurements µ + 1 σ 68.3 µ + 2 σ 95.5 µ σ 95 µ + 3 σ 99.7

Confidence interval Range of values that will contain the true mean of the population to a given probability C.I. when s is large? small? C.I. when n is large? small?

Advantages to larger sample size –More confident of true value Disadvantages of larger sample size –Money –Time –Not enough sample

z vs. t t tends to z at large n’s Values of both in tables depend on confidence level –t also depends on degrees of freedom → t for a single measurement → t for n measurements