Prelab: Molar Mass of a Volatile Liquid 100.100 Mass of dry flask and rubber sto

Question

Prelab: Molar Mass of a Volatile Liquid 100.100 Mass of dry flask and rubber stopper (g) Mass of dry flask and rubber stopper and volatile liquid after heating (g) Temperature of boiling water(C) Temperature of boiling water (K) Volume of the entire 125 ml flask (ml) Volume of the entire 125 ml flask (1) Barometric pressure (atm) 100.175 98.00 130.00 0.980 Calculations 1. What is the mass of your unknown liquid after the flask has been allowed to cool? 2. How many moles of your unknown liquid do you have? (n=PV/RT) 3. What is the molecular mass of your unknown liquid in units of g/mol? (molar mass mass/moles) 4. Your unknown liquid as a gas would occupy what volume at STP? (V-nRT/P) 5. Your unknown liquid as a gas would have what density at STP? (D-mass/V) What was the main objective of Part I of this lab? Write several sentences below

Explanation / Answer

The temperature of boiling water = (98 + 273.15) K = 371.15 K

The volume of entire 125 mL flask = (125/1000) L = 0.125 L

1. The mass of unknown liquid after the flask has been allowed to cool = 100.175 - 100.100 = 0.075 g

Explanation: Since the flask is closed with a rubber stopper, the liquid vapors will remain in the headspace, whereas after cooling, the liquid vapors condense and settle down. As a result, the mass of liquid will remain the same after heating as well as after cooling.

2. According to the ideal gas equation, PV = nRT

i.e. 0.980 atm*0.125 L = n*0.08205 L atm mol-1 K-1*371.15 K

i.e. The no. of moles of unknown liquid (n) = 4.0226*10-3 mol

3. Formula: no. of moles = mass/molar mass

i.e. 4.0226*10-3 mol = 0.075 g/molar mass

i.e. The molar mass of unknown liquid = 0.075 g/4.0226*10-3 mol = 18.645 g/mol

4. As shown in part 2, PV = nRT

STP is nothing but standard temperature (T = 0oC = 273.15 K) and pressure (P = 1 atm)

i.e. 1 atm * V = 4.0226*10-3 mol * 0.08205 L atm mol-1 K-1 * 273.15 K

Therefore, at STP, the volume of liquid as a gas = 0.090 L = 90 mL

5. Formula: density = mass/volume

i.e. The density of unknown liquid at STP = 0.075 g/90 mL = 8.333*10-4 g/mL = 0.833 g/L

The main objective is to determine no. of moles, the molar mass, volume occupied as a gas at STP and denisty of unknown liquid at STP, using the ideal gas equation, i.e. PV = nRT

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