Question

How would you calculate the number of moles and atoms in a sample of gold that had a mass of 35.12 grams?

Answer 1

1 mole=6022*10 to the 23 power of molecules (:
So, just flip it. :D

Answer 2

@jaderoman Have you got the answer?

Answer 3

nope

Answer 4

Ok. First, we need to find the moles of gold

Answer 5

Next, one mole of gold contains 6.022x10^23 Au atoms.
Therefore, if we multiply moles of Au by 6.022x10^23, we will get the number of Au atoms.

Answer 6

\(\sf 35.12g\times \large {\frac{1~mole~Au}{196.97~g}} \times \large {\frac{6.022\times10^{23}~Au~atoms}{1~mole~Au}} = ...Au~atoms\)

Answer 7

If you don't understand, I will try to explain

Answer 8

I dont understand.. i'm just really confused with this whole topic

Answer 9

@tinybookworm could you please explain

Answer 10

Sorry for letting you wait

Answer 11

First, you may know that \(\sf mass=moles\times molar~mass\), right?

Answer 12

@jaderoman Are you there?

Answer 13

yes sorry I haven't been on

Answer 14

I guess i'll continue on from tinybookworm.

What are you confused on?

Answer 15

I just don't know how to do it to be honest. I've been really behind on my schoolwork so I haven't been doing too well with it.

Answer 16

Ok, so lets start from the beginning.

The problem asks for the number and moles/atoms in a sample of gold. The only information it gives you is the mass of the sample of gold.

Answer 17

So lets define these terms, do you know what mass, moles, and atoms are?

Answer 18

Yes

Answer 19

Ok good

Answer 20

Do you what molar mass is? (its sometimes also called atomic weight/atomic mass)

Answer 21

Sorry I hope you're still there, my wifi went out. Anyways, isn't it the amount of something in a substance?

Answer 22

It describes the amount of mass per mole of a substance. If you look on the periodic table, its ypically the bottom number on the element.

Answer 23

Ohh okay

Answer 24

So I need to find the molar mass for Au?

Answer 25

Yes

Answer 26

Its 196.966543?

Answer 27

Its given on the periodic table, you just need to look for it.

Answer 28

Yes

Answer 29

okay so what do i do with that

Answer 30

The entire meaning of that number is telling you that 1 mole of Au is equivalent to having 196.97 g of Au

Answer 31

okay so does that mean i would divide 35.12 by that? to find the amount of moles it is?

Answer 32

So I want to use an analogy so you can understand this better. Say that I have a group of cars, and each of the cars of 2 people in them. If I have 8 cars, how many total people would I have?

I would have 16 people in total right? I just took the number of cars and multiplied by the number of people per car. Now, lets take this in reverse, if I had 20 people, how many cars would I have?

Its 10 cars. I just took the amount of people and divided by the number of people per car, in order to find the total number of cars.

Anyways, this car analogy is pretty much what you are doing for this chemistry (stochiometry) problem.

So just think about that for a bit, while I help someone else real quick. I'll be back in a minute or two.

Answer 33

okay.. that just confused me

Answer 34

what are the cars and people supposed to represent?

Answer 35

The car and people are like mass and moles/atoms. While the number of people per car is like the molecular mass.

Answer 36

oh okay that makes sense

Answer 37

So in order to convert between mass and moles/atoms. We need to use molecular mass as kind of an intermediary value, like I did in the car example.

Answer 38

So since I have the mass of Au, in order to get moles, I need to do this:

\[\frac{35.12\,g\,Au}{}*\frac{1\,mole\,Au}{196.97\,g\,Au} = ?\,mole\,Au\]

So I need to divide by the molecular weight in order to get the number of moles. I setup the problem in the fractional form above, so its a bit more explicit about what I'm really doing. I'm essentially using the fraction, so I can figure out whether to divide or multiply by the molecular weight. I'm dividing, so I can eliminate the mass unit (grams) and I'm left with moles as my answer.

Answer 39

To find the number of atoms you just need to know that 1 mole = 6.022 x 10^23 atoms. Then like what we did with the molecular weight above, use that as an intermediary value to convert from moles to atoms. So like this:

\[\frac{?\,moles\,Au}{}*\frac{6.022*10^{23}\,atoms}{1\,mole} = ?\,atoms\,Au\]

So the fraction tells me to multiply by the intermediary value to get atoms.

Answer 40

so for the first one i would divide 35.12 by 196.97?

Answer 41

Yep

Answer 42

and the second step you multiply.

Answer 43

But does it make sense to you how I know when to multiply/divide?

Answer 44

so i guess i would say about 0.18 moles? and i think i get it

Answer 45

and i plug that into the second equation?

Answer 46

Yep, I got 0.18 moles, and you plug that value into the first ? mark I made in the second equation.

Answer 47

I'm kind of confused on what I'm multiplying though

Answer 48

On the second equation? We are multiplying by the number of atoms in 1 mole of a substance. Thats what 1 mole is defined as, 6.022 * 10^23 units

Answer 49

so 0.18 x 6.022 x 10^23?

Answer 50

Yes

Answer 51

is it supposed to be a really big number?

Answer 52

or did i do it wrong

Answer 53

yep, it should be. A mole is 6.022*10^23, which is about the same size as the volume of water in the pacific ocean. By multiplying by 0.18 you are essentially only taking 18% of that value. So think about 18% of the pacific ocean, thats still really big.

Answer 54

1.08*10^23 atoms
Thats what I got.

Answer 55

Okay do you think you could help me with the other problems too?

Answer 56

Sure, I would start a new thread though. I'm currently helping someone else at the moment, but I'll try to help you out as soon as I'm done. If I don't get to you in time, you might be able to get help from someone else.

Answer 57

it's basically the same thing except i have to do it with table sugar, which has 3 different elements in it so I'm not really sure what to do there

Answer 58

Well instead of just one atom you have a multiple of atoms in a molecule. The only thing different is that you are dealing with molecular mass rather than atomic mass.

Answer 59

Okay I'll just have to start a new thread because I'm not quite sure how to set it up

Answer 60

@4n1m0s1ty Thank you for your explanation