Question

In a lab experiment, 7.75g of phosphorus reacts with bromine to form 67.68g of phosphorus tribromide

a)      Calculate the percentage by mass of P in phosphorus tribromide

b)      How many grams bromine reacted and how do you know?

Answer 1

a)

P + 3Br --> PBr3 (that's your reaction, right?)

Then the % P by mass will be the mole percentage. Look at your periodic table for the molar masses of P and Br.

Answer will be

(molar mass of P)/(molar mass of P + 3 x molar mass of Br) multiply by 100 to change from a decimal to a percentage.

= 7.75/(7.75+3*67.68) = 0.0367

= 3.67%

b)

Your limiting reagent is the phosphorus. Calculate how many moles of P there are in 7.75g.

There would need to be 3 x as many moles of Br reacting (we're assuming 100% conversion from P to PBr3).

Once you have the moles of Br, convert back to mass (mass = moles x molar mass)