The compound XF3 containts 65% F (mass percentage). Mw (x) = ?

The compound XF3 has a molar mass of 87.7 g/mol

The element x has an atomic mass of 30.7 u

This element can be P (phosphorus) and the compound will be PF3

Explanation:

Step 1: Data given

Compound XF3 has a mass % of 65% F

Step 2:

The sample has 65% mass percent, this is the ratio of the element (F) it's mass compared to the mass of the compound.

(Mass of F / mass of compound) * 100% = 65 %

Step 3: Calculate molar mass of the compound

F has an atomic mass of 19u (Since there are 3x F the atomic mass is 3*19 = 57 u)

The compound XF3 has a molar mass of (X+57) g/mol

⇒ with X = the molar mass of compound x

Mass % of F = 65% = (3*19g/mol) / (X+57) g/mol

0.65 = (57 / (X+57))

57 = 0.65 * (X+57)

57 = 0.65X + 37.05

19.95 = 0.65X

X = 30.7 g/mol

This means the atomic mass of the element x is 30.7 u

To control this we can calculate the mass % of F

((3*18) / (30.7 + 3*18)) * 100% = 65%

The compound XF3 has a molar mass of 87.7 g/mol

The element x has an atomic mass of 30.7 u

This element can be P (phosphorus) and the compound will be PF3