Question

BaCl2 (aq) and K2CrO4 (aq) combine in a double replacement reaction.

Answer 1

This is a double replacement reaction. You just swap the cations and anions. So the equation would be (without being balanced):
BaCl2(aq) + Na2SO4(aq)---> BaSO4 (s) + NaCl(aq)

You know what reacts together based on the basic rules and the solubility rules. There is no acid. There is no water. So the chances of this being gas forming isn't there. And its double replacement. So either it doesn't react, or its the formation of a solid. According to the solubility rules, when a sulfate reacts with a salt it IS soluble unless the salt is Ba, Pb, Ca, Sr, or Hg2. So when pair up Ba with SO4, it is insoluble, aka, A solid.

Then Balance the equation..
BaCl2(aq) + Na2SO4(aq)---> BaSO4(s) + 2NaCl(aq)

That is the complete molecular equation. Now to figure out the ionic, you need to break them all apart into individual ions (including charges and states.. So.

Ba2+(aq)+2Cl-(aq)+2Na+(aq)+SO42-(aq)->
2Na+(aq)+2Cl-(aq)+BaSO4(s)

Now the concept is you cannot break apart the solids (Only strong electrolytes dissociate in water). SO this is the correct total ionic equation. To get the net ionic equation, you must eliminate all ions that are identical on BOTH sides, otherwise known as spectator ions.

SO the net would be (after eliminating spectators):

Ba2+(aq) + SO42-(aq)---->BaSO4 (s)

All the other ions are useless, essentially, they do not take part in the reaction.