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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: galpinj on September 20, 2016, 11:41:19 PM

Title: Finding molarity through density
Post by: galpinj on September 20, 2016, 11:41:19 PM
Question is as follows:

A solution contains 10% HCl by mass in water. What is the molarity of the solution? The molar mass of HCl is 36.45 g/mol and the density of HCl is 1.05 kg/L

The answer is 2.88 M  (10/100)(1/36.45)(1.05)(1000)

I do not understand why we need to use the density of HCl when solving this question. Can't we just do the following:

(10 g of HCl / 100 g water)(1 mole HCl / 36.45 g HCl)(1 g H2O/1 mL H2O)(1000 mL H2O/1 L H2O). Wouldn't this give us the molarity for HCl?

Edit: Even if we exchanged 100 g water for 100 g solution, I don't know how we could cancel it out with density. Isn't density for HCl they provided literally kg HCl / L HCl?
Title: Re: Finding molarity through density
Post by: sjb on September 21, 2016, 02:27:19 AM
Quote from: galpinj on September 20, 2016, 11:41:19 PM
Question is as follows:

A solution contains 10% HCl by mass in water. What is the molarity of the solution? The molar mass of HCl is 36.45 g/mol and the density of HCl is 1.05 kg/L

The answer is 2.88 M  (10/100)(1/36.45)(1.05)(1000)

I do not understand why we need to use the density of HCl when solving this question. Can't we just do the following:

(10 g of HCl / 100 g water)(1 mole HCl / 36.45 g HCl)(1 g H2O/1 mL H2O)(1000 mL H2O/1 L H2O). Wouldn't this give us the molarity for HCl?

Edit: Even if we exchanged 100 g water for 100 g solution, I don't know how we could cancel it out with density. Isn't density for HCl they provided literally kg HCl / L HCl?

What is the definition of density?
Title: Re: Finding molarity through density
Post by: Borek on September 21, 2016, 02:40:37 AM
Quote from: galpinj on September 20, 2016, 11:41:19 PM
the density of HCl is 1.05 kg/L

Poor wording. This is not density of the HCl, this is density of the HCl solution.
Title: Re: Finding molarity through density
Post by: galpinj on September 22, 2016, 01:50:29 PM
Quote from: Borek on September 21, 2016, 02:40:37 AM
Quote from: galpinj on September 20, 2016, 11:41:19 PM
the density of HCl is 1.05 kg/L

Poor wording. This is not density of the HCl, this is density of the HCl solution.

Sjb, density is defined as mass/volume

Borek, I agree. After watching a couple of videos with similar questions, it is obvious that my equation is wrong in that it should be 10 g of HCl / 100 g of solution. This can be cancelled out if we have the density of the solution, not the density of HCl. I'm assuming that the density provided is meant to be the density of the solution.

Thank you everyone