How good is your GCSE-level knowledge? Test it here!

I'm revising for an aptitude test, and would like you guys to help me with some questions. Please ONLY ANSWER WITH A DETAILED EXPLANATION OF HOW YOU GOT THE RESULT. Don't just post a guess or some jibberish, and write like you're talking to an idiot.

This test only requires knowledge taught at current GCSE level. NO CALCULATORS ALLOWED!

Question 1:
Approximately 1 in 14 men over the age of 50 has prostate cancer. The level of 'prostate specific
antigen' (PSA) is used as a preliminary screening test for prostate cancer.
7% of men with prostate cancer do not have a high level of PSA. These results are known as 'false
negatives'.
75% of those men with a high level of PSA do not have cancer. These results are known as 'false
positives'.
If a man over 50 has a normal level of PSA, what are the chances that he has prostate cancer?

[Nutsinatin]

Hopefully I have the correct answer, lets find out...

Firstly the 75% is a red herring as this man has a normal reading and that only applies to a high reading, so forget about that.
We know that 7% of men with a normal reading like this man do still have cancer.
We know that 1/14 men have prostate cancer, and that 7% of men with a normal reading also have cancer.
Therefore if we multiply the chance of him having cancer (1/14) by the chance of the test giving a false negative (7/100) we get 1/200 (0.5%) chance.
To summarise: (1/14) x (7/100) = 1/200 which is the same as 0.5/100 which is the same as 0.5%.

Hopefully someone will be along to confirm if I know what I'm talking about or if I'm leading you off down the wrong path and need to take this test too!

No, that is correct!

Next one:

The Avogadro constant is 6.0 x 10^23 mol-1.
How many hydrogen atoms are there in 0.420 g of cyclohexane?
[Ar: H = 1; C = 12]

[Nutsinatin]

I'll take the bait...

Cyclohexane will have the chemical formula C6H12
It's Mr will be C6(12x6)= 72 + H12(1x12) = 12 so Mr = 84gmol^-1
So the Mr of cyclohexane will be 84gmol^-1
If n moles = Mass/Mr then n = 0.420/84= 5x10^-3mol
So we have 5x10^-3mol of cyclohexane
As the Avogadro constant is per mole of substance we need to multiply the number of moles of cyclohexane by the constant. Therefore 5x10^-3 x 6.0x^23 = 3x10^21
So we have 3x10^21 molecules of cyclohexane.
Each cyclohexane molecule contains 12H atoms, so we have 12 times as many H atoms as cyclohexane molecules, therefore 3x10^21 x 12 = 3.6x10^22
So we have 3.6x10^22 hydrogen atoms in 0.420g of cyclohexane.

I hope I'm right else I'll have to hang my head in shame for that one, although I'm not sure how clear my workings are, might have to tidy that one up when I'm not so tired!

I don't understand a word of that, can see why I'm a builder lol

[Kenzie]

Yeah, I am glazing over a bit as well.

My 14 year old has gone to bed or else I would ask him, best I don't wake him as he has GCSE mocks starting tomorrow. If he doesn't know the answers now he will be in trouble.

[Redmist]

42

That's a lot more in depth than the gcse science I did.

Closer to some of the A level questions.