[Noor]

Hi all..ok so this is due by Friday (Sept 28) and I've been trying to do these problems but everytime I attempt an answer in my online asst. it says that the answers are wrong.

So the questions that are wrong are as follows:

1) Determine the mass of a one-trillion-atom (1.0E12) sample of metallic platinum.

2) Determine the mass of two nitrogen atoms.

3) Determine the molar mass, M, of an element if the mass of a 2.55* 10^22-atom sample of the element is 5.10 g.

My answers were:

1) 3.2394 * 10 ^ -10 g.

2) 4.652166 * 10 ^-23

3) 120.4000028 g/mol

Can someone please verify those? I am pretty sure I did everything correctly..why does it say my answers are wrong?

[Padfoot]

I agree with you for 1 and 2 - they look right to me. 
For 3 though, probably a typo on your part  - I got 120.44 - unless ofcourse I'm wrong 

[Noor]

^How did you get 120.44?

My method of doing it is this:


Ok so there's a slight change: 120.40228 but I don't enter that many digits anyway..it asks for only 3 which is the 120 part..argh...why is it doing this to me..

[Padfoot]

I think our difference lies in the fact that I used a value for Avagadro's Number that is more accurate than yours (I used 6.02205*10²³).

120.44g=6.02205*10²³/(2.55*10²²)*5.10g

So we were both right 

[Yggdrasil]

You are given values in with only three sig figs, which means that your answers to #3 should have only three sig figs.  Not surprisingly, your answers are the same to three sig figs.  The small difference in your answers is due to differences in rounding.

[Noor]

Ok. Disregard the roundings...are they correct for sure?

Do I take it that there's something wrong with the system and not with my answers? I have to check with my prof. *sigh*