Hydrocarbons A and B both contain 85.7 % carbon. Their molar masses are 42 and 84 respectively.

a) Which elements does a hydrocarbon contain?
b) Calculate the empirical formula of A and of B.
c) Calculate the molecular formula of A and B.

I solved a) and b)

the solutions are
a) Hydrogen and Carbon
b) the simplest ratio is 2:1 so empiral formula is CH2

Very well answeredπŸ‘πŸ‘

For a C mass fraction of 0.857 = 12/14, the C:H ratio must be 1:2

c) Let the formula be CnH2n
For A, the molar mass is
42 = n*12 + (2n)*1 = 14 n, so
n = 3
The formula of A is C3H6

B has twice as many atoms of each.

Thanks

What is the molecular mass of carbon monoxide

c) To calculate the molecular formula, we need to find the actual number of carbon and hydrogen atoms in the molecule.

For hydrocarbon A:
Given that the molar mass of A is 42 g/mol and it contains 85.7 % carbon, we can calculate the number of carbon atoms as follows:

Mass of carbon = (85.7 / 100) x 42 g/mol = 35.994 g

Now, we can calculate the number of moles of carbon:

Moles of carbon = (mass of carbon / molar mass of carbon) = 35.994 g / 12 g/mol = 2.999 mol (approximately)

Since the empirical formula of A is CH2, we know that for every 1 carbon atom, there are 2 hydrogen atoms. So, the number of hydrogen atoms in A would be:

Number of hydrogen atoms = (number of carbon atoms) x 2 = 2.999 x 2 = 5.998

To simplify, let's round this to the nearest whole number:

Number of hydrogen atoms = 6

Therefore, the molecular formula of hydrocarbon A is C3H6.

For hydrocarbon B:
Given that the molar mass of B is 84 g/mol and it contains 85.7 % carbon, we can follow the same steps as above to calculate the molecular formula.

Moles of carbon = (mass of carbon / molar mass of carbon) = (85.7 / 100) x 84 / 12 = 6.998 mol (approximately)

Number of hydrogen atoms = (number of carbon atoms) x 2 = 6.998 x 2 = 13.996

Round this to the nearest whole number:

Number of hydrogen atoms = 14

Therefore, the molecular formula of hydrocarbon B is C7H14.

So, the molecular formulas are:
A: C3H6
B: C7H14

Hope that puts a smile on your face!

To solve part c) and find the molecular formula of hydrocarbons A and B, we need to compare their molar masses (as provided) with their empirical formula masses.

Let's first calculate the empirical formula mass for hydrocarbon A:

1. Determine the molar mass of carbon (C): 12.01 g/mol
2. Determine the molar mass of hydrogen (H): 1.008 g/mol

Using the empirical formula CH2, calculate the empirical formula mass:
Empirical formula mass = (mass of C) + 2 * (mass of H)
Empirical formula mass = (12.01 g/mol) + 2 * (1.008 g/mol)
Empirical formula mass β‰ˆ 14.03 g/mol

Now, compare the given molar mass of hydrocarbon A (42 g/mol) with the empirical formula mass (14.03 g/mol):
42 g/mol Γ· 14.03 g/mol β‰ˆ 2.998 β‰ˆ 3

So, the molecular formula of hydrocarbon A is CH2 (empirical formula) multiplied by 3:
Molecular formula = CH2 * 3 = C3H6

Now let's calculate the molecular formula of hydrocarbon B:

Using the same method, calculate the empirical formula mass for hydrocarbon B:
Empirical formula mass = (mass of C) + 2 * (mass of H)
Empirical formula mass β‰ˆ (12.01 g/mol) + 2 * (1.008 g/mol)
Empirical formula mass β‰ˆ 14.03 g/mol

Compare the given molar mass of hydrocarbon B (84 g/mol) with the empirical formula mass (14.03 g/mol):
84 g/mol Γ· 14.03 g/mol β‰ˆ 5.991 β‰ˆ 6

Therefore, the molecular formula of hydrocarbon B is CH2 (empirical formula) multiplied by 6:
Molecular formula = CH2 * 6 = C6H12

So, the molecular formula for hydrocarbon A is C3H6, and for hydrocarbon B, it is C6H12.