Consider the reaction: k2s(aq)+co(no3)2(aq)=2kno3(aq)+cos( s) what volume of 0.255 m k2s solution is required to completely react with 160 ml of 0.130 m co(no3)2?

Based off of the chemical equation given, you know that K₂S reacts with Co(NO₃)₂ in a 1 to 1 molar ratio.  That means that the number of moles of both reactants need to be equal for both of them to completely react with each other.  

You can find the number of moles of Co(NO₃)₂ by multiplying 0.130M by 0.160L to get 0.0208 moles of Co(NO₃)₂.  That means that you need to find a volume of 0.225M K₂S that contains 0.0208 moles of K₂S.  You can do this by dividing the 0.0208 by 0.255 to get 0.0816L of K₂S.  That means that you need 81.6mL of 0.255M K₂S to fully react with 160mL of 0.130M Co(NO₃)₂.

I hope this helps.  Let me know in the comments if anything is unclear.