AVOGADRO’S HYPOTHESIS – SAMPLE PROBLEMS KEY PLEASE NOTE THAT Na = Avogadro’s number = 6.02 x 1023 molecules 1. There are 3.67 x 1032 molecules of nitrogen gas in a flask at STP. How many molecules of oxygen gas would be present in the same flask? 3.67 x 1032 molecules of oxygen gas. From Avogadro’s Hypothesis, we have learned that the number of moles (or molecules) of the gas in a sample at a given volume, temperature, and pressure does NOT depend on the identity of the gas. 2. 312 L of chlorine gas at STP. What mass of fluorine gas would be present at the same volume, temperature and pressure. 312 L x 1 mole / 22.4 L x 38.0 g / mole = 529 g 2b. How many molecules would this be? 13.9 mole x Na = 8.37 x 1024 molecules 3. What mass do 4.37 x 1032 molecules of ammonia gas possess? 4.37 x 1032 molecules x Na x 17.0 g / mole = 1.23 x 1010 g 4. 156 g of ammonia gas in a flask at STP. What mass of chlorine gas would fit in the same flask? 156 g x 1 mole / 17.0 g = 9.18 moles AVOGADRO’S HYPOTHESIS: (V, T, P) NH3 = (V, T, P) Cl2 # of molecules of NH3 = # of molecules of Cl2 moles of NH3 = moles of Cl2 9.18 moles x 71.0 g / mole = 651 g 5. 211.5 g of fluorine gas at RTP. 178 g of oxygen gas in the same container at RTP. How many litres of each gas are present at RTP? 211.5 g x 1 mole/38.0 g x 24.5 L / mole = 136 L F2 178 g x 1 mole/32.0 g x 24.5 L / mole = 136 L O2 Due to Avogadro’s hypothesis, the volume that both gases occupy will be equal, since the temperature and pressure conditions are the same, and the # of moles is the same. (Check it! The number of moles of both gases is 5.57 moles) 6. A balloon holds 4678 g of He gas. What mass of hydrogen gas would it hold at identical conditions? 4678 g x 1 mole/4.00 g He x 2.02 g H2 / mole = 2.36 x 103 g (V, T, P) He = (V, T, P) H2 moles of He = moles of H2 7. What mass of Neon gas would the balloon in #6 hold (at identical conditions)? 1169.5 moles (from question 6) x 20.2 g Ne / mole = 2.36 x 104 g 7b. How do the molecules of hydrogen gas and Neon gas compare in mass? 2.36 x 104 g Ne = 10.0 2.36 x 103 g H2 mass of Ne: mass of H2 = 10: 1 ratio i.e. Neon gas’ mass is 10 times the hydrogen gas’ mass 8. How many molecules of carbon monoxide gas are present in 176 L of the gas at STP? 176 L x1 mol / 22.4 L x Na = 4.73 x 1024 CO molecules 9. A container holds 6.93 moles of ammonia gas at 42oC and 176 kPa. What mass of chlorine gas would the container hold at identical conditions? 6.93 moles x 71.0 g / mole = 491 g Cl2 10. A container holds 302 g of gas “X”. The same container holds 75 g of oxygen gas under identical conditions. What is the molar mass of gas “X”? 75 g x 1 mole / 32.0 g O2 = 2.3 moles 302 g of gas “X” = 131.3 g / mole 2.3 moles must be Xe 11. Argon has a density of 1.784 g/L. How many atoms are present in 220. mL of Argon gas? 0.22 L x 1.784 g / L x 1 mole / 39.9 g Ar x Na = 5.91 x 1021 Ar atoms 12. 361 L of C3H8 gas at STP. How many moles would exist at RTP? 361 L X 1 mole / 22.4 L X 24.5 L / mole = 395 L We know to divide by STP and multiply by RTP Since the temperature is going up, we expect the volume of the gas to get BIGGER.