How would you solve this chemistry equation?

Question

A molecule that contains only carbon and hydrogen is composed of 74.83% carbon by mass.

a) What is the empirical formula?
b) How many hydrogen atoms are present in 25.8 g of this substance?

Answer 1

a) Assume you have 100 g of this material.
Then there are 74.83g C. Find number of moles C = 74.83g/12.06g/mol = 6.237moles C
There are 100 - 74.83g H =25.17g H. Find moles H =25.17g/1.008g/mol = 25.00mol H
C6.237H25.00. Divide both values by 6.237 =
C1H4. Therefore, CH4.
b) Find the Molar Mass of CH4 = 16.08g/mol
Find number of moles = 25.8g/16.08g/mol ~1.6 moles
number of moles H = 1.6 mol CH4 x 4 mol H/1 mol CH4 = 6.4moles H
number of H atoms = 6.4mol x 6.02 x 10^23 atoms/mol = 38.4 x 10^23 or 3.84 x 10^24 atoms.

Answer 2

You have to handle these problems by converting to moles. So take a convenient mass like 100 g. The carbon content is 74.8 g and the hydrogen content is 25.2 g. Dividing the carbon mass by 12 and the hydrogen mass by 1, you get g-atoms in the ratio of about 6.2 to 25. Since the largest H to C ratio you can have is 4:1, and this is about 4 to 1, that is the empirical formula (methane).

If methane has a mole wt of 16, the number of atoms in 25.8 grams would be 4 x 25.8/16.

Answer 3

Suppose you have 100 g of this molecule.

You will have 74.83 g carbon and 100-74.83=25.17 g hydrogen.

Then find out the no. of moles of carbon using: 74.83 g / 12.01 g. (12.01 is the molar mass of C from the periodic table)

Then find out the no. of moles of H. Divide 23.17 g by 1.008 g.

Then you get the mole ratio of both. Write C and H and write the corresponding moles next to each other. Then divide by the smallest of those two numbers. (for example, if C has less moles than H, divide both numbers by C's moles).

Then you will get 1 for C or H. and whatever you get for the other one, just round it off.