In the thermite reaction 2Al + Fe2O3 -> Al2O3 + 2Fe, 5.4 g of Al is heated with 12 g of Fe2O3 (Al=27, Fe=56, O=16) and the reaction goes to completion. What mass of Al remains unreacted, in grams, to two decimal places?

JEE Advanced levelNumerical answerDifficulty 3 of 3mc154

In the thermite reaction $2 Al + Fe X_{2} O X_{3} Al X_{2} O X_{3} + 2 Fe$ , 5.4 g of Al is heated with 12 g of $Fe X_{2} O X_{3}$ ( $Al = 27$ , $Fe = 56$ , $O = 16$ ) and the reaction goes to completion. What mass of Al remains unreacted, in grams, to two decimal places?

Numerical answer type, answer in g, tolerance 0.02.

Moles: Al = 5.4/27 = 0.20;

Fe X_{2} O X_{3}

= 12/160 = 0.075.

Fe X_{2} O X_{3}

needs 0.15 mol Al, which is less than 0.20, so

Fe X_{2} O X_{3}

is limiting. Al left = (0.20 - 0.15) x 27 = 1.35 g.