Spirochete

Here's a general chemistry problem and my answer. Would anybody mind checking it? Thanks in advance. 3 Compounds have the following mass ratios of Oxygen:Nitrogen A) 1.143 g oxygen:1 g Nitrogen B) 2.286 g oxygen: 1 g Nitrogen C) 2.857 g oxygen: 1 g Nitrogen Show that these compounds obey the law of multiple proportions. To do this I compared the three compounds to each other. My answer is: A:B = 1.143 g O:2.286 g O = 0.5:1 = 1:2 A:C = 1.143:2.857 = 0.4:1 = 2:5 B:C = 2.286:2.857 = 0.8:1 = 4:5 This whole process seems so sketchy and I was hoping somebody could check this for me. Thanks

A sample of carbon dioxide gas is collected over water at 25 degrees Celsius. Vapor pressure of water (l)=23.8 mmHg. Th carbon dioxide and water occupy a volume of 1.80 L at a pressure of 783.0 mmHg. What mass of Carbon dioxide is present? I'm thinking you just use pv=nrt to solve this, and then convert moles into mass. I guess I'm just not sure what pressure to use to get an exact answer. Do you use the pressure of the CO2 only to calculate the number of moles you have? So 783 mmHg-23.8 mmHg = The pressure of CO2? Then take the number you get from that and plug it into pv=nrt? Or do I just use the pressure of 783 mmHg, making the info given about water just extraneous? Thanks for reading!