A sample of barium nitrate is placed into a jar containing water. The mass of the barium nitrate sample is 27g. Assume the water is at 20 degrees C and the resulting barium nitrate solution is saturated. What mass of water is present in the jar?

So we have Barium nitrate with a solubility of 8.7g in 100g water at 20°C.

using that relation

i. e.

8.7g (barium nitrate) = 100g (water)

1g barium nitrate = 100/8.7 g water

27g barium nitrate = (100 / 8.7) * 27

= 310.34 g

therefore,

you need 310.34g of water is in the jar.