An impure sample of Na2 SO4 has a mass of 1.56 g. This sample is dissolved and allowed to react with BaCl2 solution. The precipitate has a mass of 2.15 g Calculate the percentage purity of Na2 SO4 in the original sample.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.