[Lackydoo]

You need to prepare a 20% solution of sodium chloride using 40 g of an 80% solution of NaCl.
What mass of water is necessary to add?

I get really confused when i have questions like this one. Could anyone help me understand how I'm supposed to solve it correctly? 

[Borek]

Step by step.

How much NaCl in the 40 g of 80% solution?

The number calculated above is 20% of what mass?

[Arkcon]

I get really confused when i have questions like this one. Could anyone help me understand how I'm supposed to solve it correctly? 

Thank you for asking in just that way, because that's exactly what we do here, its all in the Forum Rules{click}

If the number were 50%, that is to say half something and the other half something else, you'd know exactly how much mass each component was.  These sorts of problems are really just the same thing.

[Lackydoo]

Here is how I'm thinking;

80% = 40g NaCl
20% = 10g H2O

What mass of water do we need to add to get 40 g of NaCl to 20%.

(20%) 40 g x 5 = 200.
80% H2O = 160.

We need to add 160 - 10 = 150 g.

Maybe I'm mixing the solutions wrongly?

[Borek]

Here is how I'm thinking;

80% = 40g NaCl
20% = 10g H2O

No, that's not correct.

Can you try to follow my questions?

[Lackydoo]

Step by step.

How much NaCl in the 40 g of 80% solution?

The number calculated above is 20% of what mass?

40 g of 80% solution = 32 g of NaCl and 8 g H2O.

To get 80% H2O and 20% NaCl we need to add 32 x 4 = 128g of water?

[Borek]

40 g of 80% solution = 32 g of NaCl and 8 g H2O.

OK

To get 80% H2O and 20% NaCl we need to add 32 x 4 = 128g of water?

Close, but not right. The final solution needs to have 128 g of water. It doesn't mean you have to add 128 g!