What mass of P4 is required to combine with 0.398 g of Cl2?

[science_rules]

Homework Statement

I have tried and tried and I can't figure out the factors to use on this:
a) What mass of P₄ is required to combine with 0.398 grams (g) of Cl₂ to produce PCl₃?
b) What mass of PCl₃ results when 0.398 g of Cl₂ is combined with the calculated mass of P₄?
I know that the equation is already balanced but I am not sure how to write the factors to get the answers. I am teaching myself, I'm not in chemistry class, so please be patient, thank you. I still haven't gotten the hang of using factors for stoichiometry problems.

Homework Equations

Equation to use: P₄(s) + 6Cl₂(g) ---> 4PCl₃ (l) Unknown: P₄ Known: 0.398g Cl₂
I know that the atomic mass of Cl₂ is 35.453 and the atomic mass of P₄ is 30.973

The Attempt at a Solution

I tried using these factors but none of them yielded the right answers: 1 mole P₄/6 mole Cl₂, 1 mole PCl₃/137.3 g PCl₃, and 1 mole Cl₂/70.906 g Cl₂ but none of them work. Like I said, I tried to solve it using the factors but I cannot figure out which to use, they are confusing to me.
I started with 0.00561 moles of Cl₂ times 1 mole P₄/6 moles Cl₂ and I crossed out the Cl₂ which would give 0.000935 P₄ which is not the answer. The answers are supposed to be 0.116 g P₄ and 0.514 g PCl₃

 

[DrClaude]

I know that the atomic mass of Cl₂ is 35.453 and the atomic mass of P₄ is 30.973

That's not correct. The atomic mass of Cl is 35.453 u and the atomic mass of P is 30.973 u.

 

[science_rules]

I just forgot to take the subscripts off

 

[Borek]

I started with 0.00561 moles of Cl2 times 1 mole P4/6 moles Cl2 and I crossed out the Cl2 which would give 0.000935 P4 which is not the answer.

0.000935 of what? Meters? Hours?

 

[science_rules]

0.000935 moles

 

[Chestermiller]

0.000935 moles

Right. So what's the molecular weight of P₄? So how many grams does 0.000935 moles of P₄ represent?

 

[science_rules]

30.97 x 4 = 123.88 amu molecular weight, 0.028 grams in 0.000935 moles of P₄ because 0.000935 moles times 30.97 =
0.028 grams

 

[Chestermiller]

30.97 x 4 = 123.88 amu molecular weight, 0.028 grams in 0.000935 moles of P₄ because 0.000935 moles times 30.97 =
0.028 grams

$0.000935\, moles \times 123.88 \frac{grams}{mole}=??$

 

[science_rules]

= 0.116 grams P₄

 

[science_rules]

Now: b) What mass of PCl₃ results when 0.398 g of Cl₂ is combined with the calculated mass of P4? Do I just multiply 0.398 g of Cl₂ times 0.116 grams of P₄?

 

[science_rules]

oops, maybe not

 

[Chestermiller]

Now: b) What mass of PCl₃ results when 0.398 g of Cl₂ is combined with the calculated mass of P4? Do I just multiply 0.398 g of Cl₂ times 0.116 grams of P₄?

That will give you something with units of grams². From your balanced reaction equation, how many moles of PCl₃ are produced for every mole of P₄ reacted?

 

[science_rules]

1 mole?

 

[science_rules]

I mean 4 moles

 

[Chestermiller]

I mean 4 moles

So, if 0.000935 moles of P₄ react, how many moles of PCl₃ are produced?

 

[science_rules]

4 moles times 0.000935 moles = 0.00374?

 

[science_rules]

0.00374 moles of PCl₃

 

[Chestermiller]

0.00374 moles of PCl₃

Correct. You're on a roll. What is the mass of 0.00374 moles of PCl₃?

 

[science_rules]

2.723 grams?

 

[Chestermiller]

2.723 grams?

Where did you get that from? What is the molecular weight of PCl3 in grams per mole?

 

[science_rules]

molecular weight of PCl₃ = 30.97376 + 35.453X3 = 30.97376 + 106.359 = 137.33276 grams PCl₃/mole

 

[Chestermiller]

molecular weight of PCl₃ = 30.97376 + 35.453X3 = 30.97376 + 106.359 = 137.33276 grams PCl₃/mole

So...?

 

[science_rules]

Do I divide 0.398 g Cl₂ by the 70.9064 g Cl₂? which would equal 0.00561 moles?

 

[Borek]

Please pay attention to what you are doing - the step you are doing now has nothing to do with chlorine, you have already correctly calculated how many moles of PCl₃ are produced, and you were trying to find the mass of

0.00374 moles of PCl3

Why do you step back to chlorine?

 

[epenguin]

My advice:
1 draw and use pictures as I mentioned in a recent post
2

This is a question in very starting chemistry. If this was a problem, if you needed a formula, or needed the indications of a teacher, if you were stuck by that question, and you are starting chemistry, let me suggest you do yourself a big favour:

Revise SIMPLE PROPORTIONS!

Whether it needs ten minutes and then it comes back, or whether you take a school arithmetic book and work through numbers of examples, do it till it is easy and obvious.

Because ALL the quantitative problems you will meet and be asked to solve for the first years in Chenistry will be really about nothing but proportions,

 

[Chestermiller]

0.00374 moles of PCl3 times 137.33 grams per mole of PCl3 gives you how many grams of PCl3?

 

[science_rules]

0.514 g of PCl₃

 

[science_rules]

My advice:
1 draw and use pictures as I mentioned in a recent post
2

thank you

 

[science_rules]

0.00374 moles of PCl3 times 137.33 grams per mole of PCl3 gives you how many grams of PCl3?

Thank you for your help!