Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: science fan on August 23, 2009, 05:55:56 AM

Title: Acid calculations
Post by: science fan on August 23, 2009, 05:55:56 AM

Hello

What mass of HCL must be dissolved in 300ml of water to give a pH= 2

I have calculated the molecular mass of HCl : M(HCL) = (1.008) + (35.45)
                                                                        = 36.458

I think the next step is to calculate the number of moles which I am having trouble with (finding/choosing the right formula).

Then I think I know that to find the mass you use the formula is m=Mn

which is the mass= the molecular mass multiplied by the number of moles.

I don't know the next steps .

Help is greatly appreciated
Thank you

Title: Re: Acid calculations
Post by: Borek on August 23, 2009, 06:04:42 AM

What is pH definition?

What HCl concentration is needed for that pH?

Title: Re: Acid calculations
Post by: science fan on August 23, 2009, 06:11:32 AM

The question doesn't include the concentration needed for the pH or definition

Title: Re: Acid calculations
Post by: Borek on August 23, 2009, 06:18:26 AM

What is pH definition.

Title: Re: Acid calculations
Post by: science fan on August 23, 2009, 06:21:00 AM

pH= -log (H+)

Sorry I don't clearly know what you are asking.

The pH is 2

Title: Re: Acid calculations
Post by: Borek on August 23, 2009, 06:37:16 AM

OK, now - take a look at the pH definition - do you know how to solve it for H⁺ concentration?

Title: Re: Acid calculations
Post by: science fan on August 24, 2009, 07:06:01 AM

pH = - log[H+] = 2

pH = 2 means that the concentration of Hydrogen ions is 10-2 there for the concentration of HCL it equals 0.01 MoL -1 (Moles/Litre).

so does that mean to find the number of moles I multiply the concentration by the volume (300/1000 = 0.3). 

n(HCL)= 0.01 x 0.3
         = 0.03

Then so to calculate the mass it is m(HCL) = nM
                                                          = 0.03 x 36.458
                                                          = 0.109 g

The mass of HCL must be dissolved is 0.109 g

Would that be the correct answer

thank you

Title: Re: Acid calculations
Post by: Borek on August 24, 2009, 07:16:19 AM

Wasn't that hard :)