Question

1. A chemist observed a gas being evolved in a chemical reaction and collected some of it for analysis. It was found to contain 80% carbon and 20% hydrogen. It was also observed that 500. mL of the gas at STP weighed 0.6695 g.

What is the empirical formula for the compound?

What is its molecular formula?

2. The ratio of an unknown gas effuses at a rate that is 0.325 times that of argon gas. Calculate the molar mass of the unknown gas in g/mol.

3. 25 g of argon are placed in a 20 L container and the pressure is determined to be 1.25 atm. What is the temperature of the gas?

4. You have 10 mL of gas in a syringe at a certain pressure and temperature. Assuming the temperature remains constant, what happens to the pressure if you depress the syringe to 5 mL?

5. A substance requires 50.0 J of heat to raise its temperature by 6.00°C. If the mass of the substance is 5.00 g, what is the specific heat of the substance?

Answer 1

1)

1st find the molar mass of gas

Given:

P = 1.0 atm

V = 500.0 mL

= (500.0/1000) L

= 0.5 L

T = 273.0 K

find number of moles using:

P * V = n*R*T

1 atm * 0.5 L = n * 0.08206 atm.L/mol.K * 273 K

n = 2.232*10^-2 mol

mass(solute)= 0.6695 g

use:

number of mol = mass / molar mass

2.232*10^-2 mol = (0.6695 g)/molar mass

molar mass = 30 g/mol

Now find the empirical and molecular formula.

we have mass of each elements as:

C: 80 g

H: 20 g

Divide by molar mass to get number of moles of each:

C: 80/12.01 = 6.6611

H: 20/1.008 = 19.8413

Divide by smallest to get simplest whole number ratio:

C: 6.6611/6.6611 = 1

H: 19.8413/6.6611 = 3

So empirical formula is:CH3

Molar mass of CH3,

MM = 1*MM(C) + 3*MM(H)

= 1*12.01 + 3*1.008

= 15.034 g/mol

Now we have:

Molar mass = 30.0 g/mol

Empirical formula mass = 15.034 g/mol

Multiplying factor = molar mass / empirical formula mass

= 30.0/15.034

= 2

So molecular formula is:C2H6

empirical formula is:CH3

molecular formula is:C2H6

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