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# Half life/decay exam style question watch

1. I have literally no idea what to do at all here:

Uranium-238 (A=238 Z=92) decays into Lead-206 (A=206 Z=82) by means of a series of decays. One nucleus of U decays eventually into one nucleus of Pb.

This means that over time, the ratio of lead atoms to uranium atoms increases. This ratio may be used to determine the age of the sample of rock.

In a particular sample of rock the ratio, number of lead atoms: number of uranium atoms = 1:2

a) Show that the ratio

number of uranium atoms left: number of uranium initially = 2:3

Assume that the sample initially contained only uranium atoms and subsequently it contained only uranium and lead atoms.

b) Calculate the age of the rock sample. The half life of Uranium-238 is 4.47x109 years.

c) The rock sample initially contained 5.00g of Uranium. Calculate the initial number N0 of atoms of uranium-238 in the sample.

Helpful hints would be more useful here than solutions. I'd like to do the answers myself but I just have no idea where to start Edit: I figured out part a) after it turns out I was just reading it wrong...
2. (Original post by CharleyChester)
I have literally no idea what to do at all here:

Helpful hints would be more useful here than solutions. I'd like to do the answers myself but I just have no idea where to start Edit: I figured out part a) after it turns out I was just reading it wrong...
For (b) - can you determine what part of uranium that was present initially in the rock is still remaining in the sample?

For (c) - what is the mass of one atom of uranium?

Hope you'll find it helpful Should you need any more help, let me know!
3. (Original post by jaroc)
For (b) - can you determine what part of uranium that was present initially in the rock is still remaining in the sample?

For (c) - what is the mass of one atom of uranium?

Hope you'll find it helpful Should you need any more help, let me know!
Ignore me i answered my own question, thanks xD
4. Well, if you know things like the mass of a proton/neutron/electron and how many there are in a particular isotope of U then you can work out through simple addition and multiplication. Or you could do it through knowing the mass of one mole of U and avagadro's number.
5. (Original post by hothedgehog)
Well, if you know things like the mass of a proton/neutron/electron and how many there are in a particular isotope of U then you can work out through simple addition and multiplication. Or you could do it through knowing the mass of one mole of U and avagadro's number.
Can't do it from the mass of protons and neutrons because, when they are combined together to make a nucleus, some of the mass is dissipated. So the mass of a 238U nucleus is not [(92 x mass of proton) + (146 x mass of neutron)]. The lost mass is known as the mass defect, and the energy to which it corresponds is known as the binding energy.

Second suggestion is the way to go
6. (Original post by Cora Lindsay)
Can't do it from the mass of protons and neutrons because, when they are combined together to make a nucleus, some of the mass is dissipated. So the mass of a 238U nucleus is not [(92 x mass of proton) + (146 x mass of neutron)]. The lost mass is known as the mass defect, and the energy to which it corresponds is known as the binding energy.

Second suggestion is the way to go
Well, when I did A level it was acceptable to do the first method. But anyway, second is much easier.

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