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Formula from Chapter 3: chemical amount from mass of a pure substance

You can use this if: you know the chemical formula of the compound

\(M_{\mathrm{\ce{MgCl2}}}\) \(= M_{\mathrm{\ce{MgCl2}}}\)    molar mass (one formula unit of \(\ce{MgCl2}\) contains one magnesium atom and two chloride atoms)

\(\ \ \ =95.21\ \frac{\mathrm{g}}{\mathrm{mol}}\)


\(m_{\mathrm{\ce{MgCl2}}}\) \(= 4.0\ \mathrm{g}\)    about a teaspoon of \(\ce{MgCl2}\)


\(n_{\mathrm{\ce{MgCl2}}}\) \(= \dfrac{m_{\mathrm{\ce{MgCl2}}}}{M_{\mathrm{\ce{MgCl2}}}}\)

\(\ \ \ =\dfrac{4.0\ \mathrm{g}}{95.21\ \frac{\mathrm{g}}{\mathrm{mol}}}\)

\(\ \ \ =0.042\ \mathrm{mol}\)