дано

m(Mg) =49 g

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m(MgO) = ?

2Mg+O2-->2MgO

M(Mg) = 24 g/mol

n(Mg) = m/M =  49 / 24  = 2.04 mol

2n(Mg) = 2n(MgO) = 2.04 mol

M(MgO) = 40 g/mol

m(MgO) = n*M = 2.04 * 40 = 81.6 g

ответ 81.6 г