What is the formula of the hydrate?

Question

the mass of beaker=74.22g , the mass of beaker and contents before heating = 96.43g , mass of beaker and contents after heating = 86.28g , and the chemical name of the anhydrous salt is barium hydroxide

I have the answer from a key but I need clarification on how to get the answer

Answer 1

Hi Allie,

Let's start off with what we know:

1) m_beaker = 74.22g

2) m_beaker and m_contents before heating = 96.43g

3) m_beaker and m_contents after heating = 86.28g

We know that we started with 96.43g - 74.22g = 22.21g of compound.

After heating, we lost 96.43 - 86.28g = 10.15g of compound due to dehydration. This part is mostly water, and so we can convert this to moles of water using the molar mass. If you do this correctly, you should get 0.56.

From this information, we know that 22.21 - 10.15g = 12.06 of the compound that remains or is in other words anhydrous. We would then use the molar mass of barium hydroxide to find the moles of this compound that we have. If you do this correctly, you should get 0.07.

This is only our empirical formula, so we need to find the molecular formula by dividing 0.56 H₂O / 0.07 Ba(OH)₂ = 8

Therefore, we know that we should have 8 H₂O / 1 Ba(OH)₂ so our final answer is Ba(OH)₂ * 8(H₂O).

Hope this helps!

-Winn