A tooth filling is made of a silver amalgam that comprises 37 mol% silver. What is the activity of the mercury, a_{(\mathrm{Hg)}}?
The activity of the mercury a_{(\mathrm{Hg)}} is the same as its mole fraction, x_{(\mathrm{Hg)}}. By definition
x_{(\mathrm{Hg})}+x_{(\mathrm{Ag})}=1
so
x_{(\mathrm{Hg})}=1-x_{(\mathrm{Ag})}=0.63
The activity is therefore
x_{\mathrm{(Hg)}}=a_{\mathrm{(Hg)}}=0.63