[satycorn]

Hi everyone! New here!
I wanted to say that I really like chemistry and I'm way better at chemistry than at maths or physics, anyway not that it has anything to do with it lol. However, I have this test tomorrow and I have been studying for quite for time, but I was absent for two days a row in chemistry and those lessions are very poorly-explained
in the book. I think I've somewhat learned it but still
there are a couple of problems that I'm puzzled at. I'll tell you guys the problems and what I tried to do, please tell me if it's right or wrong and how to correct it.
1) There are 250ml of KOH in a container, with a pH of 13. Find how much KOH is weighted to create the solution.
What I doid: First the equation: KOH->K+ + OH-
Next, we know that pOH=14-pH and pOH=-log[OH-], so log[OH-]=-1, [OH-]=0,1
I calculated the molecular mass of KOH, which 56g/mol and that's where I'm stuck .
2) The next one says "How many ml of H2SO4, C%=98% with density=1,84g/mol are needed to make 200ml a solution of H2SO4 with normality CN=0,5N
Guys I'm sorry and I know I sound selfish but I did what I could and Im really stuck.
Please let me know if you can lend a hand! Thanks!
(C%=weight of the solubled substance/(Volumexdensity),

[Hunter2]

Number 1

You already on the right way. From the molar mass get the amount of your solution, this you have to consider to your volume.

Number 2

Get the molarity of  sulfuric solution, from this you get the mass of 1 liter.

The mass you can convert to a volume by specific gravity and using the percentage number.

The whole thing you have to decrease to your given volume.

[satycorn]

Number 1

You already on the right way. From the molar mass get the amount of your solution, this you have to consider to your volume.

Number 2

Get the molarity of  sulfuric solution, from this you get the mass of 1 liter.

The mass you can convert to a volume by specific gravity and using the percentage number.

The whole thing you have to decrease to your given volume.

Hey thanks!!
But on the first problem, how do I find the mass of the solution, the formula is mass=nxM (number of moles times molar mass), and I don't have the number of moles?... Or the Molarity=numberofmoles/Volume, still no number of moles? :S

[Hunter2]

From the pH you get the normality and also the molarity of the solution. It means a value mol/l. With  m= n*M you can get the value of mass/volume [g/l]. Now decrease this to your 250 ml.

[satycorn]

From the pH you get the normality and also the molarity of the solution. It means a value mol/l. With  m= n*M you can get the value of mass/volume [g/l]. Now decrease this to your 250 ml.

But how? Isn't the Normality=mass/equivalent mass*Volume, we can find the equivalent mass and we know the volume but we still don't know the mass...

[Hunter2]

You know the molar mass of KOH. You calculated already the normality of the pH 13 solution which is 0.1 n = 0.1 mol/l . So how much is now this value in g/l?

[satycorn]

You know the molar mass of KOH. You calculated already the normality of the pH 13 solution which is 0.1 n = 0.1 mol/l . So how much is now this value in g/l?

Wait, I found the normality by calculating the concentration of OH- ions? That's not written in my book, sorry I know I made a mish mash. So if I found the normality already, I can find the mass? Which would be normality*equivalent mass*volume=0,1*56*0,2=1,12?
Sorry if I'm wrong, seriously . And English is not my native language so I hope you're not mad already.

[Hunter2]

My mother language is also not english, so no problem. So far so good, but where does the 0.2 comes from? I thought you have 250 ml?
Please use also units in calculations to avoid errors.

[satycorn]

My mother languge is also not english, so no problem. Sor far so good, but where does the 0.2 comes from? I thought you have 250 ml?

Ohhh yeah sorry it is 250ml. So am I right? So for next time, whether if I find the concentration of H+ ions in an acid, or the concentration of OH- ions in a base I've found the normality?

[Hunter2]

Yes and no. You need the molarity. How much is it in the second question?

[satycorn]

Yes and no. How much is it in the second question?

\
The what, the concentration of OH-s?
Or were you talking about the second problem? :S

[Hunter2]

The KOH question  is solved. How much gram KOH you need?

I am talking about the sulfuric acid question.

Normality is not equal molarity. Only in one case its equal. Which one?

[satycorn]

The KOH question  is solved. How much gram KOH you need?

I am talking about the sulfuric acid question.

Normality is not equal molarity. Only in one case its equal. Which one?

Umm, so I need 1,12gram KOH?
Umm, so normality is equal to molarity when the equivalent mass is equal to the molar mass, so when there is only 1 group of OH- or 1 H+? And it would be CN=m.M*V so it would be CN+CM?
About the Sulfuric question, I think I did it, can you check it?
So since we had the normality of the solution we want to get, and the formula of the normality is CN=mass/Equivalent mass*Volume, I Found the molar mass (98g/mol), then the Equivalent Mass, 98/2=49g/mol, then I found the mass, m=CN*49*0,2=4,9gram. So we have 4,9 gram in the end, am I right?
Next, since we have the concentration in percentage, and the formula is C&=[mass/volume*density]*100%, I found the volume which is V=100mass/C%*density=490/(98*1,84)=2,7l
Tell me if I'm right or wrong, and thanks a lot

[Hunter2]

Where does the 1.12 comes from? you have 0.1 n KOH and the volume is 250 ml. How much is the mass?

Second one you want to make 200 ml 0.5 n sulfuric, how can you  do it by using 2.7 liter!!

First 0.5 n is how much in molarity?

If you know this calculate the mass of it. Similar like above.

The calculated mass you convert with the specific gravity and the percentage to the volume.

Thevolume you have to decrease to the 200 ml.

[Borek]

There are 250ml of KOH in a container, with a pH of 13. Find how much KOH is weighted to create the solution.
What I doid: First the equation: KOH->K+ + OH-
Next, we know that pOH=14-pH and pOH=-log[OH-], so log[OH-]=-1, [OH-]=0,1
I calculated the molecular mass of KOH, which 56g/mol and that's where I'm stuck

From the dissociation equation you know there is one mole of [OH^-] produced per each mole of KOH. That means concentration of OH^- is identical with the analytical concentration of KOH. You don't need to care about normality/molarity conversion here.

But no, it is not 1.12 g. You still failed to calculate the result for 250 mL, 1.12 g is for 200 mL.

You need to take care of normality in the second problem. And you did, but as you have not paid any attention to units you are way off:

V=100mass/C%*density=490/(98*1,84)=2,7l

2.7 liters of acid to prepare 0.2 liters of the solution?

2.7 is actually a correct value, just the unit is wrong.

Edit: Hunter was faster.