Sciencemadness Discussion Board

Percentage

CrimpJiggler - 13-3-2012 at 12:35

When concentration is given as a percentage (i.e. %alcohol, %HCl, %acetic acid) is it always % w/v?

Nicodem - 13-3-2012 at 12:41

Quote: Originally posted by CrimpJiggler  
When concentration is given as a percentage (i.e. %alcohol, %HCl, %acetic acid) is it always % w/v?

No, it is never w/v unless the author is totally clueless about basic mathematics. Units do not cancel in weight/volume ratio.
Unless otherwise specified, it is always weight/weight.

neptunium - 13-3-2012 at 12:49

or v/v ... like my teacher used to say in 5th grade..you cant compare apples and oranges..

bfesser - 13-3-2012 at 13:07

<strong>CrimpJiggler</strong>, I suggest you read the articles linked to from <a href="http://en.wikipedia.org/wiki/Percentage_solution_(disambiguation)" target="_blank">this</a> <img src="../scipics/_wiki.png"> page. I think you will find them informative and helpful. Happy reading.

[Edited on 7/9/13 by bfesser]

gutter_ca - 13-3-2012 at 15:02

Even more confusing when it comes to acids. A 25% Phosphoric acid solution often refers to a solution made by dilution of concentrated (~85%) H3PO4 1:4 with water.

zoombafu - 13-3-2012 at 15:10

This is why everyone should use molarity.

bfesser - 13-3-2012 at 15:11

When dealing with ubiquitous acids, such as H<sub>2</sub>SO<sub>4</sub>, HCl, or H<sub>3</sub>PO<sub>4</sub>, you can often find concentration tables in handbooks, such as the <a href="http://www.hbcpnetbase.com/" target="_blank">CRC Handbook</a> or <a href="http://books.google.com/books/about/Lange_s_Handbook_of_chemistry.html?id=ln0eAQAAIAAJ" target="_blank">Lange's</a>. I suggest trying them before you try to calculate your own values, as they will only list those values that make sense (can exist).

Vikascoder - 14-3-2012 at 00:52

percentage can be v/v . on molarity w/w. generall it is mentioned on the back of chemical bottles

CrimpJiggler - 18-3-2012 at 09:09

Quote: Originally posted by Nicodem  
Quote: Originally posted by CrimpJiggler  
When concentration is given as a percentage (i.e. %alcohol, %HCl, %acetic acid) is it always % w/v?

No, it is never w/v unless the author is totally clueless about basic mathematics. Units do not cancel in weight/volume ratio.
Unless otherwise specified, it is always weight/weight.


How does that work for gas or salt solutions (i.e. %HCl in hydrochloric acid or %NaCl in brine)? I hate all this ambiguity, I wish everyone would just use molarity. So can I safely assume that when I see "5% acidity" on a bottle of vinegar, it means 5% v/v?

Nicodem - 18-3-2012 at 09:44

Quote: Originally posted by CrimpJiggler  
How does that work for gas or salt solutions (i.e. %HCl in hydrochloric acid or %NaCl in brine)? I hate all this ambiguity, I wish everyone would just use molarity. So can I safely assume that when I see "5% acidity" on a bottle of vinegar, it means 5% v/v?

Just like you were already told. There is no particular ambiguity in the use of %, because unless specified it signifies weight per weight ratio and if the ratio is volume per volume it should be specified as "vol%", "v/v" or any other such sign. The only ambiguity comes from the suspicion that the person who specified the concentration might be mathematically impaired. Unfortunately, this often is the case.

Whether it is about a gas (HCl) or solid (NaCl) dissolved in a liquid (or anything else), it has no consequences. How could it matter? For example, if you have a reagent bottle of 37% hydrochloric acid, then 1 kg of this solution, which amounts to the volume of about 0.85 L, contains exactly 370 g HCl and 630 g water. There is nothing more practical than percentages to calculate masses in the simplest way. Yet mases are not always what ones desires to know and thus percentages can sometimes become unpractical.

So, one kg of a 5% aqueous acetic acid would contain exactly 50 g acetic acid (the rest is water and other components). In this case, due to the densities of water and acetic acid being close to 1 kg/L, and assuming no other component besides water, the w/w and v/v become approximatively similar: 1 L of this solution weights approximately 1 kg and contains a bit less about 50 mL acetic acid. This is an exception though. You can not make the same approximation, for example, for H2SO4(aq), CaCl2(aq) or other such.

peach - 18-3-2012 at 10:29

There's a handy table on the rhodium backup at erowid that lists some common concentrations against their percentages, molarity and normality. <--- link 1

There are also calculators for finding the various calculators that might help. <--- Link 2

Another example. <---Link 3

Mira - 23-3-2012 at 06:31

From .125 L of 85%phosphoric acid (minimum assay 85%) how will I prepare .125 L of 35% Phosphoric acid ?

GreenD - 23-3-2012 at 07:42

M1V1=M2V2, or C1V1=C2V2.

Mira - 23-3-2012 at 08:02

Is this correct ?
Density = 1.685
Molarity of 85% = (1685 *0.85)/98 = 14.61479592
Molarity of 35% = (1685 *0.35)/98 = 6.017857143

McVc = MdVd

Vd =14.614* 0.125/6. 018 = 0.30354769


So add .125L of 85% to approx 2.5 times of water to get 35%






[Edited on 23-3-2012 by Mira]