# I just started Chemistry need help please

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.

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.

How is density defined?

so-crates said:
How is density defined?
what do you mean?

what do % represent here? Mass?

rikku said:
what do you mean?

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.

rikku said:
what do % represent here? Mass?

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.

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