Determining a Molecular Formula from an Empirical Formula
1. A compund has an experimental molar mass of 78g/mol. Its empirical formula is CH. What is its molecular formula?
The idea here is to see how many CH groups you have or put another way (CH)x where you want to determine x.
So if x is 1, what would the molar mass be? It would be 12 for C + 1 for H = 13 for the CH. If x = 2, then the molar mass would be 26. If x = 3, the molar mass would be 3 x 13 = 39. So what time 13 = 78 (or close to it).
13 times 6 =78
CH=78g/mol
but the relative atomic mass of C=12 and H=1
therefore the molecular mass would be
(12+1)n=78 g/mol
13n = 78 g/mol
n=6
the molecular formula is C6H6 .
To determine the molecular formula from the empirical formula, we need to know the molar mass of the compound. In this case, we are given that the experimental molar mass of the compound is 78g/mol.
First, let's calculate the empirical formula mass of the empirical formula, CH. The empirical formula mass is the sum of the molar masses of all the atoms in the empirical formula.
The molar mass of carbon (C) is approximately 12g/mol, and the molar mass of hydrogen (H) is approximately 1g/mol. Therefore, the empirical formula mass of CH is:
Empirical formula mass = (1 * molar mass of C) + (1 * molar mass of H)
= (1 * 12g/mol) + (1 * 1g/mol)
= 12g/mol + 1g/mol
= 13g/mol
Now, we need to determine the ratio between the experimental molar mass of the compound and the empirical formula mass. This will give us the number of empirical formula units present in the molecular formula.
Ratio = Experimental molar mass / Empirical formula mass
= 78g/mol / 13g/mol
= 6
The ratio of 6 indicates that there are 6 empirical formula units in the molecular formula. Therefore, the molecular formula can be obtained by multiplying the subscripts of the elements in the empirical formula by 6.
Molecular formula = (Empirical formula)6
= CH6
= C6H6
Therefore, the molecular formula of the compound with the empirical formula CH and an experimental molar mass of 78g/mol is C6H6.