# Help!! Chem exam tomorro...

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#1
I've got a chem exam tomorro morning..left revisinon a bit late I know. Been donig a past paper and have come a across a question which i know the method to answer it is really easy but have completely forgotten and don't want to loose easy marks 'cause maths is one of my best subjects...
Question is...

A beryllium compound has the percentage composition by mass shown in the table. The relative atomic masses of the elements present in the compound are also shown. Calculate the formula of the compound.

Element Ar %
Be 9.0 12.9
C 12.0 17.3
O 16.0 69.8

I'm not looking for an answer just the method is fine...thank you...

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oops my table isn't very clear...If you need me to put it better I will...
0
14 years ago
#2
ok,

Step 1: you divide each of the percent atomic masses by its respective mass number, so for Be, (12.9/9). do it for each and you'll get a set of numbers.
Step 2: you divide each one of the new lot by the lowest of that lot.You should get near whole numbers. and this is the empirical formula. assign each of the whole numbers to their respective elements. there you go. hope this doesn't seem too complex. you can post the answer and see if i got the same thing
0
14 years ago
#3
Element Ar %
Be 9.0 12.9
C 12.0 17.3
O 16.0 69.8

ok..

1) set out table...

Mass (%) Be= 12.9 C = 17.3 O=69.8
molar mass Be = 9 C=12 O=16
Moles = ??

2) work out moles using moles = mass/molar mass

Moles (Be) = 12.9/9 =1.43333333
Moles (C) = 17.3/12 =1.441666
Moles (O) = 69.8/16 =4.3625

This is the ratio of the elements in the compounds..

So, it will be BeCO3
or BexCxO3x

good luck!
0
14 years ago
#4
dude, she didnt want the answer.
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#5
ok..one Be one C and three O...empiricle formula...BeCO3... thank you hun...
0
14 years ago
#6
(Original post by icecreamman)
dude, she didnt want the answer.

shurrup. She wanted the method...which I've shown.
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#7
how do you do subscripts on here??
0
14 years ago
#8
[ sub ] text [ /sub ]

(without the spaces)
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#9
trhanks...so want more questions which i don't know the answers to? haha there is no point in resitting this exam tomorro...I haven't done any revision n I did this work like a year ago...drank way too much alcohol since then to remember!!
0
14 years ago
#10
Do you know about ... bonding?
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#11
what about bonding? the different types n stuff?
0
14 years ago
#12
What types of different bondings are there?

> Intermolecular
> Intramolecular ?
0
14 years ago
#13
intermolecular:
covalent
dative covalent
metallic

Intramolecular:
van der waals
dipole dipole
dispersion forces
hydrogen bonding

i may be wrong
0
#14
ok..well never heard of intramolecular...or intermolecular bonding for that matter...

I know about intermolecular forces between molecules...

there are perminant - perminant dipole forces which occur between polarised molecules

instantaneous - induced dipoles which occur between all because of the elctrons moving around

and have forgotten the other one...:s

Is that the kind of thing you were after?
0
14 years ago
#15
hydrogen bonding is the other intermolecular force

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(Original post by Magician)
shurrup. She wanted the method...which I've shown.
yh, but she didnt want an answer. she could do it herself.
0
14 years ago
#16
Yep. - I did badly on these questions last year.
0
14 years ago
#17
(Original post by Magician)
intermolecular:
covalent
dative covalent
metallic

Intramolecular:
van der waals
dipole dipole
dispersion forces
hydrogen bonding

i may be wrong
You are either wrong or the other way round.
0
6 years ago
#18
magician where are you unit 1.6 notes !!!???!!!???
0
6 years ago
#19
(Original post by .XD)
magician where are you unit 1.6 notes !!!???!!!???
lool magician posted this 8 years ago
0
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