GS-2 Physical Sciences Question 39

[rsonia167876]

I don't understand how the number of moles of P4 = 1/4

according to the equation P4 (s) + 6Cl2 → 4PCl3

i thought it would have been 1 mole of P4, therefore, when i did the problem i got C as the answer. Can someone please help me get clear on this?
thank you

[sharada.ba5352]

The problem gives you 68.75g of PCl3. I found the molar mass of PCl~135g

68.75g/135g = approx 1/2 mole of PCl3

P4 + 6Cl2 --> 4PCl3

1/2 mole PCl3 x (1 mole P4/4 mole PCl3) = 1/8 mole P4 x 6.022x10^23

6.022/8 approx 0.75 so the answer is 0.75x10^23 (A)

You have 1/2 mole of PCl3 and you have to consider the coefficient 4 in front of PCl3 above. That gives you 1/8 mole of P4. Hope this helps.

[rsonia167876]

Thank you Sharada:), it make sense now.

[jwmenard3521]

i think i've missed something. where did the equation come from?

[uprince1005536]

I think what she was trying to say as i understand it was

1mole P4 --> 1/2 mole PCl3
4mole P4--> ?

setting up the math multiplying by avagadros number should get you the answer.
e.g
(1/2 mole PCl3) X (1 mole P4)/(4 moles P4)