The reaction of 2-bromo-2-methylpropane and naphthalene is stoichiometrically in a mole ratio of 1:1.
Notice that in the question, the moles of 2-bromo-2-methylpropane placed in the flask wasn't mentioned. if I assume that it was 1.24 moles then;
1 mole of naphthalene reacts with 1 mole of 2-bromo-2-methylpropane
x moles of naphthalene reacts with 1.24 mole of 2-bromo-2-methylpropane
x = 1 * 1.24/1
= 1.24 moles of naphthalene
Molar mass of naphthalene = 128.2 g/mol
Mass of naphthalene = 128.2 g/mol * 1.24 moles of naphthalene
Mass of naphthalene =159 g
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