Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Ciubba on February 17, 2015, 08:09:56 PM

Title: Reverse Osmosis
Post by: Ciubba on February 17, 2015, 08:09:56 PM

Assuming that seawater at 20°C is a 1.1 Mc solution of NaCl (aq), calculate how many liters of seawater are required to prduce 15 liters of freshwater at 20°C with an applied pressure of 75 bar.

Π=RTMc, so Mc=(75 bar)/ [(.08314 L bar K^-1 mol^-1)*293 K], which simplifies to 3.07882 Mc.

I need to know how many liters of water would have to be removed from seawater in order to produce that level of concentration, so Mc₁V₁=Mc₂V₂, where Mc₁ is 1.1 and Mc₂ is 3.07882. For convenience, I'm going to make V₂=1, so that I will know how much seawater produces one liter of fresh water. V₁=2.7989 L.

I need 15 liters of fresh water, so I multipied that value by 15 to get 42 L; however, the book lists the correct answer as being 23 L. Where did I go wrong?

Title: Re: Reverse Osmosis
Post by: Borek on February 18, 2015, 02:55:40 AM

Van 't Hoff factor.

Title: Re: Reverse Osmosis
Post by: Ciubba on February 18, 2015, 05:48:19 PM

Quote from: Borek on February 18, 2015, 02:55:40 AM

Van 't Hoff factor.

Hmm, where would I need to consider that? Mc1V1=Mc2V2 is fine as NaCl has the same i factor as NaCl and I see no other place involving Mc.

Title: Re: Reverse Osmosis
Post by: Borek on February 19, 2015, 02:50:04 AM

Quote from: Ciubba on February 18, 2015, 05:48:19 PM

Mc1V1=Mc2V2 is fine as NaCl has the same i factor as NaCl and I see no other place involving Mc.

What about the other equation you listed?

Title: Re: Reverse Osmosis
Post by: Ciubba on February 19, 2015, 05:44:57 PM

It can't be Π=RTMc as I intentionally solved for Mc instead of M as the concentration of the salt water was given in colligative molar, so that just leaves the multiplication by 15, right?