CHE 315 – Lecture 5 9/9/05 Statistics. Quiz 2 1.An iron ore was analyzed by dissolving a 1.1324-g...
-
date post
19-Dec-2015 -
Category
Documents
-
view
216 -
download
0
Transcript of CHE 315 – Lecture 5 9/9/05 Statistics. Quiz 2 1.An iron ore was analyzed by dissolving a 1.1324-g...
Quiz 2
1. An iron ore was analyzed by dissolving a 1.1324-g sample in concentrated HCl. After precipitation, filtration and washing, the residue was ignited to give 0.593 g of pure Fe2O3 (159.69 g/mol). What was the % Fe (55.847 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)
n321i
i
ii
x....xxxx
n
xx
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
1n
)xx(s i
2i
Confidence intervalRange of values that will contain the true mean of the population to a given probabilityC.I. when s is large? small?C.I. when n is large? small?
n
tsx
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’sValues of both in tables depend on confidence level– t also depends on degrees of
freedom
→ t for a single measurement
→ t for n measurementsns
xt
s
xt
xz