[sweetshelly]

This is the question:
How many phosphorus atoms are contained in 8.75 moles of H2P2O7? Put your answer in E-format with 1 number before and 2 numbers after the decimal place. e.g., 6.02 x 1023 would be input as 6.02E23.


the answer is : 1.05E25 (1.05 × 1025)

I really have no idea where to start. Please someone help me

[Albert]

I really have no idea where to start. Please someone help me

Just to start: how many atoms of phosphorous can you see in this molecule/formula, H₂P₂O₇?

[sweetshelly]

There is 2 atom of P in the compound. And then what. I'm sorry if I sound lazy or really stupid but honestly I have no idea what to do.

[Borek]

How many molecules per mole?

[sweetshelly]

there is 6.02 x 10^23 molecules per mole.

I try mutliplying the 2 mole of P to (6.02 x 10^23) but then I got 1.204 x 10^24.
This is not the fright answer. Please give me another hint.

[Albert]

Theoretically, how many moles of phosphorous would ONE molecule of H₂P₂O₇ produce?

H₂P₂O₇ -> x H + y P + z O

Then, solve the proportion: 1 : y = 8.75 : Y

Finally, you'll get your answer by multiplying Y by Avogadro's number.

[Borek]

Please give me another hint.

You have many moles of substance, not one.

[Gerard]

how many atoms in the equation...
if you have difficulty in the questin i suggets you observe thier units....
recall the you are given an amount of 8.75 moles of H2P2O7(see the units mole) and recall that avogadro's number has a unit of atoms(or molecules)of a certain compund or an element per mole
by dimensional anaylsis you can get the desired quantity right?
look in mathematics if you are given "A" and the desired unit is "B" yet you are gicen a constant which is X "B"/"A"(where x can be any integer)
multiplying both:
"A"(x "B"/"A") both "A"'s cancel outs right?
so:
by dimensional analysis
 8.75 moles of H2P2O7 Xavogadros number(molecules or atoms per mole) both the mole unit cancels out! leaving you with atoms or molecules
i would like to stress out that the concept of dimensional analysis is a powerful tool in solving chemistry problems so better master it its very easy
thank you
and keep up tje passion for chemistry
-artificial student

[sweetshelly]

Thank you for everyones kind help. I understand the concept now.

P has two mole so:
(2 x 8.75)(6.02 x E23) = 1.05 E23

This is a problem on our review. and usually the test is very much a like the review just my teacher change the compound, formula, and numbering around. I need to get a A tomorrow to make an A average for the semester. And there is only 20 question total on the test so I can't affford not knowing how to solve a question.

Once again thanks so much.