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My question is the following:

$2~\mathrm{g}$ table salt is added to $0.5~\mathrm{m^3}$ water whose salt concentration is $10~\mathrm{mg\,L^{-1}}$. Compute the salt concentration of the mixture in ppm. 

I am fairly new to chemistry actually this is my first time taking a chemistry course of this level so I'm having quite a hard time. Therefore, if anyone could recommend some websites where I could receive help or even videos, such as khanacademy that would be greatly appreciated!

As for the following question all I know is that $2~\mathrm{g} = 2000~\mathrm{mg}$ and ppm is calculated by:

$$\text{ppm} = \frac{\text{grams of solute}}{\text{grams of solution}}\times 1000000 $$

However, im confused as to how much grams of solute and grams of solution there will be in the mixture after adding the $2~\mathrm{g}$ of salt?

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  • $\begingroup$ You should start by figuring out how much solute there is in the original solution - this you can find out by multiplying the concentration by volume. It's safe to assume that the volume will not change substantially. $\endgroup$ – IT Tsoi May 16 '16 at 12:43
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Let's think about it :

You have the formula of the mass concentration $C_m$:

$C_m = \frac{m}{V}$,

where $m$ is the mass in grams of solute and $V$ the volume in liters of the solution.

A first step for you would be to calculate the mass of solute already in the solution : your solution has a volume of $0.5~\mathrm m^3$, which is $V=500~\mathrm L$, with a massic concentration of $C_m=10~\mathrm{mg\cdot L^{-1}}$. Therefore, there is already a mass

$m=C_m~\times~V = 10\cdot10^{-3} \times 500 = 5~\mathrm g$

of salt in your solution.

To this mass, you add $2~\mathrm g$ of salt, so the mass of solute in your solution is now $m=5~\mathrm g + 2~\mathrm g = 7~\mathrm g$.

So, at end you have a mass $m=7~\mathrm g$ of salt.

I let you compute the mass of solution, knowing that $1~\mathrm L$ of water weights $1~\mathrm{kg}$.

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  • $\begingroup$ Thank you so much for your help! Also do you happen to have any suggestions for some resources that could help me better understand this topic. As for the mass of the solution I get 500 000 grams? does this make sense? As my final answer I get 14 ppm. $\endgroup$ – ChewyMasta May 16 '16 at 13:18
  • $\begingroup$ For the mass of the solution, you indeed have $500~L = 500~kg=500\cdot 10^{3}~g$, and the value in ppm seems right. As for the ressources I can't really help you. I suggest you do as many exercices as you can from the ressources available in you course. KhanAcademy also looks like a good ressource for you. Also, please consider accepting my answer if it helped you. $\endgroup$ – Frédéric May 16 '16 at 13:38

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