Question #5b51f

1 Answer
Jan 24, 2018

Here's one way to do it.

Explanation:

Problem:

Assume you are reacting 20.0 g of carbon and 20.0 g of oxygen to form carbon dioxide. What is the mass of excess reactant used and what mass of excess reactant is unused?

Solution:

M_r:color(white)(mmmll) 12.01 color(white)(mm)32.00Mr:mmmll12.01mm32.00
color(white)(mmmmmmm) "C" color(white)(m)+ color(white)(m)"O"_2color(white)(m) →color(white)(m) "CO"_2mmmmmmmCm+mO2mmCO2
"Mass/g:"color(white)(mmm)20color(white)(mmml)20Mass/g:mmm20mmml20
"Amt/mol:"color(white)(mll)1.67color(white)(mm)0.625Amt/mol:mll1.67mm0.625

You calculated that:

  • The amount of "O"_2O2 is 0.625 mol
  • The amount of "C"C is 1.67 mol
  • "O"_2O2 is the limiting reactant
  • "C"C is the excess reactant

Then

"Moles of C used" = 0.625 color(red)(cancel(color(black)("mol O"_2))) × "1 mol C"/(1 color(red)(cancel(color(black)("mol O"_2)))) = "0.625 mol C"

"Mass of C used" = 0.625 color(red)(cancel(color(black)("mol C"))) × "12.01 g C"/(1 color(red)(cancel(color(black)("mol C")))) = "12.5 g C"

"Mass of unused C = 20.0 g - 12.5 g = 7.5 g"

Therefore,

  • Mass of excess reactant used = 12.5 g
  • Mass of unused excess reactant = 7.5 g