[zafar31]

How many litres of (NH3) [NH][/3] (the volume is measured in standart conditions) must be dissolved in 200 g of a 10% solution NH4OH to obtain 15% solution of the ammonium hydroxide?

Please show elaborate solution! And sorry I could not use symbols properly!

[zafar31]

answer must be 6.9 litres If you want to check

[Arkcon]

I'd like to welcome you, zafar31: to the Chemical Forums.  According to the Forum Rules{click}, we want to see your attempt, so please click on that link, read the contents, and comply with our rules, so we can help you. 

Can you begin to talk about something you recently learned that is pertinent this question? Can you start with the reaction of the reagent you're using, with water?

[zafar31]

I found one formula:
M1 * x1+ m2 * x2=x3(m1+m2)
m - mass of substance
x - concentration in percent
x3 - final percentage

So in this question m1 is unknown, as NH3 consists of only NH3 x1=100%, m2=200g x2=10%,
x3=15%. from this I GOT answer 11,7 g which means 0,69 moles of NH3 as 1 mole equals to 22,4
litres I assumed that answer will be {15.456} and it is incorrect.
 I have solved many exercises in this way and they were correct.

[zafar31]

If you can find mistake (or if you think that my way of solving is totally wrong) please let me know what should I change!!!

[Borek]

Around 15.5 L of ammonia looks OK to me.

[mjc123]

I agree with 6.9 L; I think you are ignoring the difference between ammonia and ammonium hydroxide. To get a solution 15% in NH₄OH, you add x g NH₃ and get (nominally) x*35/17 g NH₄OH in solution, so that
20 + x*35/17 = 0.15*(200 + x)

[Borek]

Good point.