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Questions about percent composition/simplest formula

Hello, I have a few questions about Percent Composition and Simplest Formula. My teacher gave us this hand out with some questions on it that'll be on a test and didn't explain how to do some of the questions. If someone could explain how to do the questions that would be great. Thanks. I know this is a lot to ask so I'd be happy with someone answering even just one of the questions.

4. The percent, by mass, composition of sugar is 51.5% oxygen, 42.1% carbon and 6.4% hydrogen. If you had a 7.00g sample of sugar, what mass of carbon would be present in the sample?

8. Oxygen and hydrogen always react so that 16.00 g of oxygen and 2.00 g of hydrogen combine to produce 18.00 g of water. If 10.00g of oxygen was reacted in a container with 10.00g of hydrogen:
(a) How much hydrogen would be needed?
(b) How much hydrogen would remain unreacted?
(c) How much water would be produced?

10. When 1.61 g of aluminum is burned in oxygen, 3.04g of aluminum oxide is formed.
(a) What mass of oxygen will react with 25.3g of aluminum?
(b) What is the percent composition of aluminum oxide. Determine this in two different ways. (I know how to find percent composition, but only one way.

11. When 9.20g of magnesium bromide is decomposed, 8.00g of bromine is formed.
(a) What mass of magnesium is also formed?
(b) What mass of magnesium will be produced from the decomposition of 53.8g of magnesium bromide?

Reply 1

Beverooni

First, I always convert the % to grams (assuming you have 100%) check this in the exam because sometimes you may just get a ratio! You always divide by 100 so if you are given ratios - add them up and then divide by this. Another tip - if you see grams and are told an element then you immediately have n=m/Mr (moles is EVERYTHING in chemistry!!).
for 4. Sugar is:
C6.........H12......O6 (I make a table with the equation/compound):
42.1%....6.4%....51.5% of 7g so find the % of 7g
7x0.42...7x0.064..7x0.515
= 2.947g = 0.448g = 3.605g (add them up! = 7g) :tongue:

Reply 2

username1445490

For number 4 if the % if carbon by mass is 42.1% then the mass of carbon in 7.00g of sugar will be 42.1% of 7.00.

Reply 3

Beverooni

For the last question; always write the equation. You will know the molar coefficients then-
........MgBr2 == Mg2+ + 2Cl- (yes I have written it as if enthalpy of soltn!)
m = 9.2g........ X ...+...8g (doesn't matter because its the molar ratios!)
Mr= 184.113.....24.3.....NA
n=m/Mr so
n=9.2/184.113
= 0.05 I CAN SEE MgBr2 TO Mg2+ IS A 1:1 MOLAR RATIO. THIS MEANS THEY REACT IN EQUAL QUANTITIES so:
If MgBr2 has n:0.05 then Mg2+ will too!
If n=m/Mr then m=nMr = (0.05)(24.3) = 1.215g :smile: