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March 29, 2024, 11:52:11 pm

Author Topic: URGENT CHEM 1/2 MAGNESIUM OXIDE HELP  (Read 836 times)  Share 

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withez

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URGENT CHEM 1/2 MAGNESIUM OXIDE HELP
« on: March 26, 2019, 05:25:13 pm »
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Hi!
Just did magnesium oxide practical (the one where you burn magnesium in the crucible).
Got the following results:
Mass of Magnesium ribbon= 0.291g
Mass of Oxygen reacted= 0.152g
Mass of Magnesium Oxide Powder= 0.443g
How would I go about determining percentage composition and empirical formula of magnesium oxide from these results?
Need to be able to do this for this coming Thursday so all help very much appreciated
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whys

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Re: URGENT CHEM 1/2 MAGNESIUM OXIDE HELP
« Reply #1 on: March 26, 2019, 07:13:34 pm »
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Hi!
Just did magnesium oxide practical (the one where you burn magnesium in the crucible).
Got the following results:
Mass of Magnesium ribbon= 0.291g
Mass of Oxygen reacted= 0.152g
Mass of Magnesium Oxide Powder= 0.443g
How would I go about determining percentage composition and empirical formula of magnesium oxide from these results?
Need to be able to do this for this coming Thursday so all help very much appreciated

Okay, so to calculate percentage composition, you would do the mass of the element over the mass of the compound multiplied by 100 = percentage composition. You do this for magnesium and oxygen. For example, for the percentage composition of magnesium, you would do (0⋅291)/(0⋅443)×100. For oxygen, you do the same, but with the mass of oxygen. Multiplying it by 100 gives it the percentage form of the amount of the certain element in the compound. Hopefully this makes sense!

Empirical formula is a little more complicated (but not once you know how to do it!).
These are the steps:
1. Divide each of the masses by the molar mass (just use the mass on the pt)
2. Divide each one by the smallest number you get.
3. Multiply accordingly to get a certain ratio (you may have to do this step more than once).
4. This ratio is used to determine the empirical formula.
This video should be a big help.

If you let me know your answers, I can also cross check for you to make sure they're right.
Good luck!!!
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dashnog

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Re: URGENT CHEM 1/2 MAGNESIUM OXIDE HELP
« Reply #2 on: April 27, 2019, 07:58:38 pm »
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Percentage composition is easy: divide mass of the elements over the mass of the compound and multiply by 100.

Empirical formula: take the masses of the elements and divide by their molar masses (given by the periodic table) to determine how many moles there are of each element.
Then, divide both elements' number of moles by the smallest of the two to get approximately whole numbers.
Once you have approximately whole numbers (if they aren't, you can multiply them to scale up e.g. 0.5 Fe, 1.5 O -> 1 Fe 3 O, therefore FeO3) you will be able to write the empirical formula.

Additionally, for future reference, to get the molecular formula, you have to know the molar mass of the molecule and the molar mass of the empirical formula.
Divide the molar mass of the molecular formula by the molar mass of the empirical formula to get an approximately whole number, which will function as a multiplier.
Multiply the empirical formula and you will get the molecular formula. (e.g. CH2O = 30g/mol, molecular formula = 60g/mol -> 60/30 = 2, therefore CH2O becomes C2H4O2 (Acetic acid)).
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