The 'Tricky Questions' thread

[charmanderp]

I thought it'd be good to have a thread where everyone can post questions which they find difficult or made a mistake/almost made a mistake on because of a certain trick in wording, etc. That way we don't have to go looking for them ourselves and attune ourselves to these types of questions before the Unit 4 exam, covering all bases.

I'll start. This one had me stumped for about a minute after one of my very smart friends asked me for help. Not so much as difficult but the technique/calculations required aren't something I've seen before. It's MC but I'll pose it as short answer:

"A water bath to be used in the study of reactions at 45°C is prepared by mixing hot tap water (82°C) with 87.0 g of water at room temperature (25°C).

Calculate the mass of 'hot tap water' required to bring the sample to 45°C."

Ans: 47g (2 sfs)

I'll edit the answer in here later. Maybe we can compile all of the questions here too.

[Nobby]

"A water bath to be used in the study of reactions at 45°C is prepared by mixing hot tap water (82°C) with 87.0 g of water at room temperature (25°C).

Calculate the mass of 'hot tap water' required to bring the sample to 45°C."

I'll give it a go

Energy required to bring 87.0g of water to 45deg = 87.0 x (45-25) x 4.18
                                                                               = 7273.2 J
Energy released per gram of hot tap water = 4.18 x (82-45)
                                                                     = 154.66 J
Therefore mass of hot tap water required = 7273.2/154.66
                                                                   = 47g

By the way where'd you get this question?

[charmanderp]

That's right! Good stuff. I found it in NEAP 2010 but edited the numbers.

[Lasercookie]

Edit: Beaten. Too fast Nobby

I'll have a shot at the question.

So we can only add a certain mass of water that is 82 degrees to change the temperature. 

How much energy does it to raise water by 20 degrees?


So we need 7273.2 J worth of hot water. It'll decrease from 82 degrees to 45 (change of 37 degrees). How many grams will this be?

[charmanderp]

Come on guys! Get posting!

EDIT: Just did the NEAP 2010 MC section. Seems to be quite a good paper.