What changes and remains the same when $25\mathrm{.}00mL$ of a salt water sample is diluted to $100\mathrm{.}00mL.$

Explain why a $35\mathrm{.}00mL$ sample and a $70\mathrm{.}00mL$ sample of a water obtained from a fish tank have the same phosphate concentration? How are these two solutions similar and how are they different?

Determine the molarity for each of the following solutions:

a$.,0\mathrm{.}888$mol of $CoC{l}_2$ in $0\mathrm{.}645L$ of solution