Suppose a lab group reports a percent yield of sand of 105%. is it really possible to collect more sand than was originally present? what is a possible explanation for the “extra” product? do not consider calculation mistakes; think about the procedure used.

The maximum percent of yield should be 100%, so It wasn't possible to make a higher than 100% yield like 105% in this problem. If that happens, it will contradict the law of conservation of energy as you can't create mass.

The increased mass might be produced by impurities. One of the examples would be if you rinse the container and left some water. The water is not calculated as a substrate but will increase the results weight. 
There also possibilities that some gas in the air react and produce a solid/liquid material. If you don't weight the gas it would seem that the mass increased.

The correct answer is b

Since there is the same amount of agn03 in both silver chloride and silver nitrate they produce the same amount of moles. therefore silver chloride produces the same amount of silver nitrate, so your answer is 15.0 moles of silver nitrate. i hope this !