Ammonia has the chemical formula "N{H_3}" and it's molar mass can be found as (atomic weights can be found in the periodic table)
"{M_{N{H_3}}} = {M_N} + 3{M_H}""{M_{N{H_3}}} \\approx 14.007[{{\\rm{g}} \\over {{\\rm{mol}}}}] + 3 \\cdot 1.008[{{\\rm{g}} \\over {{\\rm{mol}}}}] = 17.031[{{\\rm{g}} \\over {{\\rm{mol}}}}]" The number of moles can be found as
"{\\nu _{N{H_3}}} = {{{m_{N{H_3}}}} \\over {{M_{N{H_3}}}}}" and in each mole there are "{N_A} \\approx 6.022 \\cdot {10^{23}}[{{\\rm{1}} \\over {{\\rm{mol}}}}]" molecules. Thus the number of molecules in our sample
"N = {N_A}{{{m_{N{H_3}}}} \\over {{M_{N{H_3}}}}} \\approx 6.022 \\cdot {10^{23}}[{{\\rm{1}} \\over {{\\rm{mol}}}}]{{45.5[{\\rm{g}}]} \\over {17.031[{{\\rm{g}} \\over {{\\rm{mol}}}}]}} \\approx 1.61 \\cdot {10^{24}}"
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