Question

A student gets 9.92g chalcone dibromide (MW 458 g/mol) from 0.039 moles of chalcone (starting material), calculate

a) # moles of chalcone dibromise

b) percent yield

Answer 1

Number of moles of chalcone dibromide , n = mass/molar mass

= 9.9 g / 458(g/mol)

= 0.022 moles

The molecular formula of chalcone is C₁₅H₁₂O

The molecular formula of chalcone is dibromide C₁₅H₁₂Br₂O

The balanced reaction is : C₁₅H₁₂O + Br₂ ----->  C₁₅H₁₂Br₂O

Molar mass (g/mol) 208.6 458

From the above balanced reaction ,

1 mol of  chalcone produces 1 mole = 458 g of  chalcone dibromide

0.039 moles of  chalcone produces M g of  chalcone dibromide

M = ( 0.039 x 458 ) / 1

= 17.86 g ---> This was the theoretical mass

Given actual mass of  chalcone dibromide is 9.92 g

So percent yield = ( actual yield / theoretical yield) x 100

= ( 9.92 / 17.86) x 100

= 55.5 %

Therefore the required percentage is 55.5 %