Question

This lab will require you to determine the number of moles of carbon dioxide in a flask. This will require you to know the volume of the flask, the temperature of the gas inside it, and the pressure of the gas. Give a one-to two sentence explanation of how each of these things will be determined.

(a) Assume for the moment that air consist of 80.00% nitrogen and 20.00% oxygen by volume. Use these figures and typical values for temperature and pressure (1.000 atm, 25.0 ºC) to calculate the density of air.

(b) This calculation affords a good approximation of the density of air at sea level. However, your lab is at an altitude approximately 777 meters (2550 feet). Knowing this, will the density of air in you lab be higher or lower than the value you just calculated? Why?

(c) Besides altitude, what else affects the atmospheric pressure? Where could you find the actual atmospheric pressure at the time of the experiment?

Answer 1

(a) Suppose the volume of air sample is: 100 mL.

Volume of N2 is 80 mL and volume of O2 is 20 mL.

Moles of N2 present : n = P* V/ R* T = 1 atm * 0.08 L/ ( 0.082 Lit atm mole^-1 K^-1 * 298 K) = 0.00327 mole

Mass of N2 : moles * molar mass of N2 = 0.00327 mole * 28 g/ mole = 0.09184 g

Moles of O2 present : n = P* V/ R* T = 1 atm * 0.02 L/ ( 0.082 Lit atm mole^-1 K^-1 * 298 K) = 0.000818 mole

Mass of O2 : moles * molar mass of N2 = 0.000818 mole * 32 g/ mole = 0.02618 g

Total mass of the sample of air : (0.09184 + 0.02618) g = 0.1180 g

Density = mass/ volume = 0.1180 g/ 100 mL = 0.00118 g/ mL

(b) As altitude increases from sea level, the air pressure decreases that increases volume of air. But the mass does not change. So, the density decreases with altitude.

(c) Besides altitude, temperature and composition of air affects the atmospheric pressure.