Chemistry Lab questions

Question

1.) You measured the volume of a gas to occupy 2.5 L, but you read the units wrong and it really occupied 2.5 mL. What was your percent error? 2.) A 5.00 g sample of hydrated barium chloride, BaCl2 · nH2O, is heated to drive off the water. After heating, 4.26 g of anhydrous barium chloride, BaCl2, remains. What is the value of n in the hydrate's formula?

Answer 1

1) Percent errors represent how off you are from the actual value. This is the formula.

 

|A - T|/A

 

"A" means the actual value, which is 2.5 mL. The "T" is the theoretical value, which is what you read. Before it's safe to subtract numbers, they must be in the same units, so convert 2.5 L to mL. That would be 2500 mL, and this is what to plug in for "T."

 

|A - T|/A

|2.5 - 2500|/2.5

2497.5/2.5 = 999

999 x 100% ≈ 1.0 x 10⁵%

 

This is quite a large percentage, but that's understandable because there's a huge difference between a milliliter and a liter.

 

2) The hydrated barium chloride sample used to weigh 5.00 grams, but then it weighed 4.26 grams. It went down by 0.74 grams because it lost water. Therefore, that sample had 0.74 grams of water. The value of "n" is asking how many moles of water there are for every mole of BaCl₂. Convert 0.74 grams of water to moles. There are 18.02 grams per mole for water

 

0.74 grams x (1 mole/18.02 grams) = 0.04 moles H₂O

 

To find out how many moles of water there are for every mole of BaCl₂, we need to find the moles of that compound that are left. There are 4.26 grams of barium chloride left. The molar mass of this compound is 208.23 grams per mole.

 

4.26 grams x (1 mole/208.23 grams) = 0.02 moles BaCl₂

 

This sample had 0.02 moles of BaCl₂ and 0.04 moles of H₂O. 0.04 is twice as much as 0.02, so this means that for every mole of BaCl₂, there are 2 moles of H₂O, so the value of n = 2.

 

I hope this helps, and good luck with your other chemistry adventures!