[Amnesiac]

Okay I joined up just to ask this. I've only worked in one lab and we order the regular 68-70% HNO3. When we say to make a 5% HNO3 solution, we'll do 5 mL of the 68% and dilute to 100mL using DI water. Now I understand this is really a solution that is less than 5% HNO3, but we refer to it as such and when I get picky about it, people say ' well its 5% conc HNO3 ' .

Do other labs practice this?

[Archer]

No other lab would get picky about it because this is a bench reagent where the solution is approximately 5% nitric acid and usually used in excess in its application.

You don't know that it's less than 5% because you don't know know the precise starting concentration.

What you are describing is a standardised 5% nitric acid solution which is rarely needed.

If you have to be exact about it then you should purchase the desired solution or dilute the concentrate to about 6%, measure the amount of acid using a primary standard and then dilute accordingly.

It's the same as 1M NaOH as a bench reagent.

You could weigh 3.997 grams into 100 ml but would this give the correct molarity to such high precision?

[Borek]

Actually - thanks to the different densities of these solutions - your recipe (in ideal case, assuming stock solution of 68%) produces solution that is 4.66 %. That's close enough to 5% to be labeled this way. As Archer wrote, bench reagents are only approximate, as long as they are within 10% or even 20% of the labeled concentration they are in most cases perfectly OK.

Exact theoretical volume of the 68% stock solution required is 5.4 mL.

[billnotgatez]

Is there an added symbol sometimes used to indicate approximate

for instance in math
~5% to me means
 about 5%

although after looking up math symbols I am not sure I am using ~ wrong
Maybe I should use ≈

[Borek]

Is there an added symbol sometimes used to indicate approximate

None that I am aware of - or perhaps I should say, none that I am aware of being generally accepted.

~5% to me means
 about 5%

That would be my understanding as well.

[billnotgatez]

I see it would be difficult to label it 5% HNO3 ± 10%
 

As an after thought
I take it that it is not common to label it 5% HNO3 ± 1%
 

[Borek]

I see it would be difficult to label it 5% HNO3 ± 10%
 

As an after thought
I take it that it is not common to label it 5% HNO3 ± 1%
 

Yes, that was 10-20% relative to the value on the label.