2.85 x 10-3 moles of an unknown compound has a mass of 0.206 g. The compound could be:

Calcium sulfide (CaS)

Explanation:

From the question

Number of mole (n) = reacting mass (R)/molar mass (m)

n = R/m Equation 1

make m the subject of the equation

m = R/n Equation 2

Given: R = 0.206, n = 0.00285 moles.

Substitute these values into equation 2

m = 0.206/(0.00285)

m = 72.3 g/mol

From the above, the compond with molar mass close to 72.3 g/mol is Calcium sulfide (CaS)

i believe the answer is true.