Weighing the Atmosphere

For the life of me I can't figure out this question. I can barely comprehend the subject material.

This was a chart, but it copy pasted like this:

Gas
Volume (%)
Molecular Weight (approximate)
Weight %
Nitrogen
78.000
28
75.00
Oxygen
21.000
32
23.00
Argon
00.930
40
1.3
Carbon dioxide
00.035
44
0.05
Average Atmosphere

100.000

29

100.00

*Percentages do not add  up to 100 due to rounding, approximation, and ignoring water vapor and all minor and trace gases.
*Weight % is found by multiplying the volume (%) by the molecular weight of that gas and dividing by the average molecular weight of the atmosphere.


Weighing the Atmosphere

At sea level, the weight of 1 cm2 of air is approximately 1 kilogram. This value, 1kg/ cm2    is also known as the atmospheric pressure.  For this analysis, we will assume a featureless Earth without continents.  This will simplify but not significantly alters your calculations.

Question 2:  How much does the atmosphere weigh over each square meter of surface?  Express this in tonnes (units of 1000 kg).  Recall the conversion factors:  104 cm2 = 1m2, 1 tonne = 1000 kg = 106 g. 
Answer:

Question 3:  How much does the atmosphere weigh over each square kilometer (km2)?  Recall that 106 m2  =  1 km2 .
Answer: