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Offline PseudoDoctor

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Normal solutions question
« on: May 22, 2023, 04:50:14 PM »
Hello, I'm an MD preparing herself to start a PhD in molecular biology, last time I really studied chemistry was in HS, but I do need to cover some basics for the admision test. I've been reviewing Normal solutions, I thought I had them, but then I did some excercises and I didnt get them right, and Im not sure where the problem is, would anyone be kind enough to help?

These are the questions: (translated from spanish, sorry if there are any syntax mistakes)
1. What normality will have a 600ml solution that has 45g of hydrochloric acid?
For this one I used a MW of 36.458g/mol, and my result was 0.486N (pEq = 36.458 g/Eq, N=75g/L) The correct answer was 2.05N

2. How many grams of solute is needed to prepare 1 litre of 0.1N of H2SO4?
For this one I used a MW of 98g/mol, and my result was 1.66g (pEq = 16.33g/Eq) The correct answer was 4.9g

Thank you!!

Offline Aldebaran

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Re: Normal solutions question
« Reply #1 on: May 22, 2023, 06:14:57 PM »
If you Google for normal solutions you will quickly find several reliable explanations of how it is calculated and in particular how it is different from the more usual molarity as a measure of concentration. If you still have a problem with your calculations after reading these explanations please do come back here for some more help.
You certainly need to get this straight if you’re contemplating a PhDin molecular biology.

Offline PseudoDoctor

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Re: Normal solutions question
« Reply #2 on: May 22, 2023, 06:30:35 PM »
I read about them and did some other excercises with no problem, these are the ones I couldn´t get right, but I'm not sure of where the mistake is so I can correct it.

Offline Aldebaran

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Re: Normal solutions question
« Reply #3 on: May 22, 2023, 06:56:28 PM »
Is this a genuine enquiry?? Giving benefit of the doubt…
Ok so take your first example which is simple because it’s a monoprotic acid. How many moles is 45g of HCl? These moles  are dissolved in 600cc so how many moles would be dissolved in 1000 cc? That gives you molarity which in this example is the same as normality.
You are having a difficulty either with the arithmetic or the concept. If you follow my steps/questions above you should get the answer quoted in your initial post. If you don’t get that answer please explain how you are doing your erroneous calculation so we can see where you are making the mistake.

Offline Borek

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Re: Normal solutions question
« Reply #4 on: May 23, 2023, 03:04:03 AM »
1. What normality will have a 600ml solution that has 45g of hydrochloric acid?
For this one I used a MW of 36.458g/mol, and my result was 0.486N (pEq = 36.458 g/Eq, N=75g/L)

You got it reversed. Concentration is amount over volume, not volume over amount.
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Offline PseudoDoctor

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Re: Normal solutions question
« Reply #5 on: May 25, 2023, 11:40:46 AM »
You got it reversed. Concentration is amount over volume, not volume over amount.

Thank you, that gave me the right result. I do have a question, I used g/Eq/g/L so the result would be Eq/L, if it should be reversed, how does the clearance(?) work?

Offline Borek

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Re: Normal solutions question
« Reply #6 on: May 25, 2023, 04:14:12 PM »
[tex]\frac{\frac{g}{Eq}}{\frac{g}{L}} = \frac{\frac{1}{Eq}}{\frac{1}{L}} = \frac{\frac{1}{Eq}}{\frac{1}{L}} \times \frac L L = \frac{\frac L {Eq}}{\frac L L} = \frac L {Eq}[/tex]
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