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Topic: Need to check the answer for another moles, mass calculation  (Read 9914 times)

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Offline busi6292

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Hello all again,

A mixture os 8g iron and 4g sulphur is heated and the elements react to form iron II sulphide/ How much iron will be left over at the end of the reaction.

I calculated the moles for each symbol and did 0.14 X 88 = 12.57 for iron and 0.125 X 88 = 11 for sulphur therefore the answer is 1.57g, am I right?

Thanks again.

Offline sjb

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Re: Need to check the answer for another moles, mass calculation
« Reply #1 on: July 30, 2009, 01:08:48 PM »
Hello all again,

A mixture os 8g iron and 4g sulphur is heated and the elements react to form iron II sulphide/ How much iron will be left over at the end of the reaction.

I calculated the moles for each symbol and did 0.14 X 88 = 12.57 for iron and 0.125 X 88 = 11 for sulphur therefore the answer is 1.57g, am I right?

Thanks again.

I'm not quite sure where your "0.14 x 88" or "0.125 x 88" come from, can you post again, with units, perhaps?

Offline busi6292

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Re: Need to check the answer for another moles, mass calculation
« Reply #2 on: July 30, 2009, 05:23:45 PM »
Hello here is what I have..this is so confusing because each question is worded slightly different.

             Fe    +      S     ------->   Fe S

RAM       56            32                   56 + 32 = 88

MOLES    8/56        4/32                 8 and 4 being the mass given in the Q

ANSWER 0.14         0.125

Don't really know from here so I thought

0.14 X 88 (the RAM At other sider) = 12.57g for Fe

0.125 X 88 = 11g

Difference is 1.57g as it asks how much is left over?

Thanks!

Offline sjb

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Re: Need to check the answer for another moles, mass calculation
« Reply #3 on: July 31, 2009, 02:15:53 AM »
Hello here is what I have..this is so confusing because each question is worded slightly different.

             Fe    +      S     ------->   Fe S

RAM       56 g/mol           32 g/mol                  56 + 32 = 88 g/mol

MOLES    8 g/56 g/mol        4/32                 8 and 4 being the mass given in the Q

ANSWER 0.14 mol        0.125 mol (SJB)

Everything seems fine until here, so...

You have 0.14 mol of iron and 0.125 mol of sulfur. Which reagent is in excess, according to your reaction equation?

Offline busi6292

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Re: Need to check the answer for another moles, mass calculation
« Reply #4 on: July 31, 2009, 05:02:57 AM »
The iron is in excess.

Offline sjb

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Re: Need to check the answer for another moles, mass calculation
« Reply #5 on: July 31, 2009, 12:56:50 PM »
OK, so (based on our other conversation I'll perhaps harry things along a bit)

How many moles of iron are you in excess by?

What is the mass of this amount of iron?

Offline busi6292

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Re: Need to check the answer for another moles, mass calculation
« Reply #6 on: July 31, 2009, 05:55:51 PM »
Well 0.14/0.125 = 1.12 so 0.12 moles in excess.

0.14 - 0.125 = 0.015 in excess.

Was that right to your qs?

Offline sjb

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Re: Need to check the answer for another moles, mass calculation
« Reply #7 on: August 01, 2009, 03:12:37 AM »
First answer - not quite. If I had 2 apples and 3 oranges, and you took away apples and oranges in equal numbers until I had no more of one or other fruit, would I really have 0.5 (3/2 - 1) oranges left?

Instead, as you have realised in the second sentence, I'd have 3-2 = 1 orange left, so in the actual question you'd have 0.015 mol of iron in excess.

So, what is the mass of 0.015 mol of iron?

Offline busi6292

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Re: Need to check the answer for another moles, mass calculation
« Reply #8 on: August 02, 2009, 07:53:08 PM »
Okay so mass = moles X ram 

0.015 x 56 = 0.84g.

Offline sjb

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Re: Need to check the answer for another moles, mass calculation
« Reply #9 on: August 03, 2009, 02:08:31 AM »
Sounds fine to me. I'd perhaps not round off the number of moles of iron that there were initially so early  - I can see why you've done it (2 sf and all that), but in that case why did you not round off the moles of sulfur, too?

Offline busi6292

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Re: Need to check the answer for another moles, mass calculation
« Reply #10 on: August 03, 2009, 08:47:13 AM »
Brilliant thanks!

Probably because doing the method was done in a duration of 5 days!

Offline sjb

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Re: Need to check the answer for another moles, mass calculation
« Reply #11 on: August 03, 2009, 01:04:57 PM »
Probably because doing the method was done in a duration of 5 days!

Well, such is the nature of collaboration. Don't forget, there is more than just your town, or even timezone on the internet. I'm sure if we had been sitting next to one another in the library or similar then it would have been over within an hour or so.

Offline busi6292

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Re: Need to check the answer for another moles, mass calculation
« Reply #12 on: August 03, 2009, 08:08:54 PM »
Yes I know but I was just telling you why I didn't round the moles to the same significant figures as other calculations.

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