The answer is 7.65*10^(-3) "moles" of Pb^(2+) ions were present in the sample.
The balanced chemical equation is:
PbCl_(2(aq)) + Zn_((s)) -> ZnCl_(2(aq)) + Pb_((s))
Notice the 1:1 mole ratio between Zn and PbCl_2; this means that one mole Zn will react with 1 mole of PbCl_2.
You know that 7.65*10^(-3) moles of Zn had reacted after one day, which automatically means that the exact number of PbCl_2 moles had reacted as well. The number of PbCl_2 moles is equal to the number of moles of Pb^(2+) ions, since
PbCl_(2(aq)) -> Pb_((aq))^(2+) + 2Cl_((aq))^(-)
The complete ionic equation looks like this:
Pb_((aq))^(2+) + 2Cl_((aq))^(-) + Zn_((s)) -> Zn_((aq))^(2+) + 2Cl_((aq))^(-) + Pb_((s))
The net ionic equation is
Pb_((aq))^(2+) + Zn_((s)) -> Zn_((aq))^(2+) + Pb_((s))
This is a single replacement reaction. Since Zn is more reactive metal than Pb, the Zn ions will completely replace the Pb ions present in the solution.
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