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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Greek0000 on August 11, 2019, 11:40:36 AM

Title: Amount of solute in a Molarity Concentration.
Post by: Greek0000 on August 11, 2019, 11:40:36 AM
I am trying to solve for grams in solute of lactic acid concentration of 0.10 M C3H6O2

notice the example and my answer was 7.4 grams of lactic acid (see attachment)
Title: Re: Amount of solute in a Molarity Concentration.
Post by: Borek on August 11, 2019, 11:58:24 AM
Pure nonsense - you can't speak about amount of substance without knowing volume of the solution. Throw away this book (or whatever it is).
Title: Re: Amount of solute in a Molarity Concentration.
Post by: sunkal on August 11, 2019, 04:24:12 PM
Greek0000,

The setup shown in the attachment under NOTE inside the box is: 0.10 M C3H6O2 x 74 g mol C3H6O2 / 1 mol C3H6O2. You should realize that this happens to be a totally wrong setup. If you do what is called dimensional analysis, you can realize it.

First, notice that mol (in 74 g mol) and mol (in 1 mol) will cancel leaving g. M stands for mol / L. So, the final unit left is g mol / L. It is not just g, as it says in the final answer.

Secondly, the correct setup for the conversion factor is: 74 g C3H6O2 / 1 mol C3H6O2. This comes from the molar mass. 74 g mol in the numerator is completely wrong.

Multiplying the molarity by molar mass will never give any useful quantity. One needs to know the volume in L. Only then a suitable dimensional analysis will occur. The equation to be used is n = M V and then the resulting n can be converted to g using the molar mass.

This is why Borek refers to it as pure nonsense.
Title: Re: Amount of solute in a Molarity Concentration.
Post by: Vidya on August 22, 2019, 01:52:28 AM
Quote from: Greek0000 on August 11, 2019, 11:40:36 AM
I am trying to solve for grams in solute of lactic acid concentration of 0.10 M C3H6O2

notice the example and my answer was 7.4 grams of lactic acid (see attachment)

A  question like this in which molarity of the solution is given and  volume is not given and we need to calculate moles or mass of the substance , then volume of the solution can be assumed to be as one liter.
So here moles equals to molarity multiplied by volume which is 1 liter.
Title: Re: Amount of solute in a Molarity Concentration.
Post by: sunkal on August 22, 2019, 08:17:33 PM
Quote from: Vidya on August 22, 2019, 01:52:28 AM
A  question like this in which molarity of the solution is given and  volume is not given and we need to calculate moles or mass of the substance , then volume of the solution can be assumed to be as one liter.
The question should give the volume; otherwise it is not a properly worded question. One should not have to make any such assumption.
Title: Re: Amount of solute in a Molarity Concentration.
Post by: Vidya on August 22, 2019, 08:47:44 PM
There are plethora of questions in which we are using this assumption if volume is not mentioned.