percent solutions - need help please

[Guest dadobi]

It has been a couple years since I have had drug calculations - would someone please remind me of the steps I need to answer the following questions

 

A 7% glucose solution is made up of what % glucose and what % water??

 

A 20% NaCl solution is made up of what % NaCl and what % water??

 

thanks so much in advance

[Skye]

For solutions it's a weight-for-weight percentage. You would normally see this written as '7% w/w' on the bottle, for the glucose solution. This solution will then contain 7% glucose, and 93% water, by weight. But it won't equal the same percentages in volume, or in moles.

[pulkit]

Solutions are not always w/w, they can be v/v as well (Eg. Ethanol solutions are always v/v) when the solute is a liquid, also there are w/v solutions.

 

7% glucose and 20% NaCl are most probably w/v solutions. w/v is one of the most popular ways to keep track of concentration, its also called strength of a solution.

[Primarygun]

dadobi, if it's volume, solute/solution

Be careful, the solution=solvent + solute

This all my little knowledge, don't scold me if I am wrong:P

[Skye]

pulkit, I assume it's w/w for solutions unless it says otherwise, this is the most common I've seen. Ethanol I've seen usually w/w, i.e. most of the ethanol around the lab used for cleaning and that will be 95% w/w. But it may just be my crazy part of the world, and it wouldn't surprise me if the glucose and NaCl were w/v.

[Kedas]

weight/volume has an unit so it can't be in %

[pulkit]

weight/volume has an unit so it can't be in %

 

True. I must acept that I am wrong.

[sfpublic]

Hello,

 

I am a tiny bit confused about this w/w term. Could you please clarify it with a few examples so that I can understand it?

 

thank you

[RyanJ]

Hello' date='

 

I am a tiny bit confused about this w/w term. Could you please clarify it with a few examples so that I can understand it?

 

thank you[/quote']

 

Ok... let see if I can explain it (I am now known for my explaining abilities )

 

Ok, in the examples remember they must all add up too 100% so...

 

Lets go through an examaple:

 

Question:

A solution is composed of 33.3% Nitric Acid, 21.6% Hydrochloric Acid and the rest is water. Calculate the percentage of water in the solution.

 

Answer:

Now, here is a rule, all percentages in this case must MUST add up too 100% so...

 

[math]33.3 + 21.6 + x = 100[/math]

 

[math]54.9 + x = 100[/math]

 

[math]x = (100 - 54.9)[/math]

 

[math]x = 45.1[/math]

 

So the percentage of water is 45.1%

 

Your examples will work the same way just remember, they just all add up to 100%

 

Here is another incase you need a bit ore practice.

 

Question:

 

A composition of an alloy requires between 0.95% too 2.1% Carbon and the rest pure Iron. Calculate the percentage of Iron needed if the Carbon content is considered too be 1.3%.

 

Answer:

Now, here is a rule, all percentages in this case must MUST add up too 100% so...

 

[math]1.3 + x = 100[/math]

 

[math]x = (100 - 1.3)[/math]

 

[math]x = 98.7[/math]

 

So you need 98.7% Iron assuming that the Carbon content is 1.3%

 

Good luck!

 

Cheers,

 

Ryan Jones

[sfpublic]

Thank you for your clear answer to the w/w question.

 

 

I had forgotten that I posted it until 10 mins ago. It was one of those I-can-wait questions.

 

Anyway, thanks

 

 

andy