Suppose 1.2 grams of calcium hydroxide are dissolved in 1.75 L of an aqueous solution. How do you determine the following: hydroxide ion concentration, hydronium ion concentration, calcium ion concentration, pH and poH?

1 Answer
Jun 1, 2017

Answer:

#"Concentration"="Amount of stuff"/"Volume of solution"#

Explanation:

And generally, we report #"concentration"# with units of #mol*L^-1#.

And so #[Ca(OH)_2]=((1.2*g)/(74.09*g*mol^-1))/(1.75*L)=9.26xx10^-3*mol*L^-1#.

Clearly, #[HO^-]=2xx9.26xx10^-3*mol*L^-1#. Agreed? Because #[Ca(OH)_2]# speciates in solution to give #Ca^(2+)#, and 2 equiv of hydroxide ion.

And thus #[Ca^(2+)]=9.26xx10^-3#, #[HO^(-)]=1.85xx10^-2*mol*L^-1#.

#pOH# is simply a measure of hydroxide ion concentration, and in aqueous solution we know that #pH+pOH=14#.

#pOH=-log_10[HO^-]=-log_10(1.85xx10^-2)=1.73#

#pH=14-1.73=12.27#.

For another example of this sort of problem, see here.