# I just started Chemistry need help please

1. Oct 23, 2004

### rikku

I just started Chemistry need help please!!!

Help me write equation for the following problem
A sample substance is 15% copper, 35% aluminum, 50% iron by mass. Density of copper = 8.92, aluminum=2.70, iron=7.86. What is the density of the sample?
I have some ideas but want to check.

2. Oct 23, 2004

### Dooga Blackrazor

I'm guessing, I haven't taken Chemistry yet: Multiply each percentage by the density of the substance. For example 0.35 x 8.9 - then add all your resultant numbers.

3. Oct 23, 2004

### so-crates

How is density defined?

4. Oct 24, 2004

### rikku

what do you mean?

5. Oct 24, 2004

### rikku

what do % represent here? Mass?

6. Oct 24, 2004

### so-crates

I mean, what is the definition of density? That might be a good place to start and might give you some insight into the problem. I am trying to follow the method of my namesake: The answer to the question is already in your mind, it just needs to be discovered.

7. Oct 25, 2004

### Chaotic42

Yes. When it says 50% by mass, it means that in a 10g sample, 5g of the mass is iron.

I like the way you think, so-crates.

8. Oct 27, 2004

### theriddler876

Help me write equation for the following problem
A sample substance is 15% copper, 35% aluminum, 50% iron by mass. Density of copper = 8.92, aluminum=2.70, iron=7.86. What is the density of the sample?
I have some ideas but want to check.

assume you have a 100 gram sample so you would have 15 grams of copper 35 of al and 50 grams of Fe, now density is mass over volume so 8.92 g/cm^3 so do dimensional analysis and see how much of the 100 grams the aluminum takes up, then add that for each value so you should get what all of them take up individually so then you get the 100 grams you assumed you had, and divide by the added values that you got