\(n_{NaOH}=\dfrac{20\cdot10}{100\cdot40}=0.05\left(mol\right)\)
\(CH_3COOH+NaOH\rightarrow CH_3COONa+H_2O\)
\(0.05....................0.05\)
\(C_{M_{CH_3COOH}}=\dfrac{0.05}{0.05}=1\left(M\right)\)
\(b.\)
\(n_{CH_3COOH}=0.075\cdot1=0.075\left(mol\right)\)
\(2CH_3COOH+A\rightarrow\left(CH_3COO\right)_2A+H_2\)
\(0.075.................0.0375\)
\(M_A=\dfrac{0.9}{0.0375}=24\)
\(A:Mg\)