Question

chem based math.

first one: http://prntscr.com/cfs1c6

There are two oxides of copper. One oxide has 20% oxygen, by mass. The second oxide has a smaller percent of oxygen than the first.
What is the probable percent of oxygen in the second oxide?
Express your answer using two significant figures.

Answer 1

We can reason it this way.
Most ionic copper compounds have Cu++, so it's a good starting point.
Its oxide is therefore CuO, with molecular mass (63.54+16.00)=79.54.
Percentage of oxygen = 16.00/79.54=20.13%,. "exactly" the same proportion given.

Now according to Wiki, there are 4 possible copper oxides.
https://en.wikipedia.org/wiki/Copper_oxide

If the one sought for has a smaller percentage of oxygen, what would that be?

Answer 2

Hello, sorry for disappearing on you. The site seems to like kicking my icon off my own posts...

Well, all I know is that it's less than...

Oh

Answer 3

@mathmate I'm not quite sure this information would be readily available on a quiz/exam, which I am studying for :(

Answer 4

But I believe everything but copper peroxide is less in Oxygen percentage?

Answer 5

@kittiwitti1
Very true, I was supplying you with extraneous information.
You should be able to solve the same problem without the link.

Remember I said the compound that correspond to Cu++ is CuO.
If the other oxide has less oxygen, that means proportionately more copper.
Would that be Cu+, or Cu+++, or Cu++++? Hint: There is only one possible choice, irrespective if the corresponding oxides exist or not.

Answer 6

Oh, I see.
Hmm... one second

Answer 7

Copper (I) oxide has a higher ratio in terms of Cu, so I would assume that one

Answer 8

Exactly! Let not the question scare you! lol

Answer 9

I ... was right? :o

Answer 10

Awesome hahah

Answer 11

:)

Answer 12

For the other problem, if the metals are in ratios
p:q:r, where p,q,r are given.

To get a given mass, M, of the alloy, you would need x units of each, namely

px+qx+rx=M

and solve for x.

Answer 13

Sorry about that!

Answer 14

Ah, I see. Thank you for the information, it was a great help @mathmate :)

Answer 15

You're welcome! :)

Answer 16

additional note:
To make sure the total mass add up to M, you need to have
px of the first metal, qx of the second metal, and rx of the third.
Add up the numbers to make sure you get M as the total mass.

Answer 17

@mathmate I did this and the answer was incorrect...
http://prntscr.com/cftp3t

Answer 18

Well, I don't know what I was doing before, but this time around I calculated each respective element's masses and then found their percentage out of the whole mass. Then I multiplied that by the 1.20 kg (in grams units though)

Answer 19

In chemistry and physics, it is VERY important to pay attention to the units.

Whenever you give an answer, ask yourself what is the unit of your answer, and whether it corresponds to the expected unit.

It is a pity that the computers are "dumb". If it was an answer on paper, you HAVE to write 600g in which case it would be correct. The way the computer works, it is not encouraging students to care about units by omitting them, which is BAD training.

So you will have to give it extra discipline to ask yourself if the units correspond.