equation

[sarahswaheli]

Hi, I can't get this for the life of me...
Does anyone know what equation or where I would start with this I thought D=mv, but it doesn't work out...

A solution of 35.00% zinc nitrate has a density of 1.3678 g/cm3. What is the volume of a sample of this solution that contains 25.00g of zinc nitrate?

Thanks

[Borek]

You must calculate mass of the solution first - and you will do it using definition of mass percentage (check for example http://www.chembuddy.com/?left=conce...ass-percentage). Then you have to convert mass to volume using density definition.

Best,
Borek

[sarahswaheli]

Hi, I'm sorry I still don't understand where to put the 35.00%

I got 0.00977 mL is that correct?

[charco]

Your answer is clearly wrong!

Just think of how big 1cm3 is (1cm3 = 1 ml) ..... now consider how big 0.00977 mL

You would not be able to see this quantity of solution!

OK lets start from first principles and discount a few fallacies:

If the density is 1.3678 g/cm3 this means that every cm3 has a mass of 1.3678 g

Now the density of water = 1g per 1 cm3

therefore 1 cm3 of solution also contains 1 - 1.3678 g of solute = 0.3676g

If you require 25g then you require 25/0.3676 cm3 = 68cm3

But this wouldn't take into account any change in volume of the solvent!

Now a 35% solution means (usually) 35% weight/volume ratio.

In other words 100cm3 contains 35g of solute

You need 25g therefore you need 25/35 x 100cm3 =71.43cm3

But again this doesn't take into account change in volume of solvent on production of solution.

So?

If 35% w/v solution has a density of 1.3678g/cm3 then this means that in every 1 cm3, 35% of the mass (1.3678g) is due to the solute...

So in 1cm3 of solution there is 35/100 x 1.3678g of solute = 0.4787g

OK?

So if you want 25 g solute then you need 25/0.47873 cm3 solution = 52.22cm3

Capichi?

[Borek]

Charco - you are reaching your left ear with your right hand. No idea why.

c%w/w = m_solute/m_solution * 100%

we know c%w/w, we know the mass of solute, we need mass of solution:

m_solution = m_solute/c%ww * 100%

m_solution = 25.00/35.00 * 100 = 71.43 g

density definition

d = m_solution / V_solution

so

V_solution = m_solution /d

V_solution = 71.43/1.3678 = 52.22 mL

Result the same, calculations three times shorter.

And density changes have nothing to do with the question - you can deal with the masses and the given density only.

c%w/w stands for mass percentage
m_solution, m_solute - masses
d - density
V-solution - volume

Best,
Borek

[charco]

Borek - you misunderstood my post!

I was showing how NOT to calculate it in the first two instances - these are the common mistakes that people make - perhaps I should have made that clearer. I was trying to explain why people approach problems like this incorrectly. If you see in the first two parts I state "this does not take into account changes in volume on dissolution" (or WTTE)

The actual calculation is only the third part.

Sorry if I mislead anyone! 8)