Percentage concentration to molar concentration

Chemistry and homework help forum.

Organic Chemistry, Analytical Chemistry, Biochemistry, Physical Chemistry, Computational Chemistry, Theoretical Chemistry, High School Chemistry, Colledge Chemistry and University Chemistry Forum.

Share your chemistry ideas, discuss chemical problems, ask for help with scientific chemistry questions, inspire others by your chemistry vision!

Please feel free to start a scientific chemistry discussion here!

Discuss chemistry homework problems with experts!

Ask for help with chemical questions and help others with your chemistry knowledge!

Moderators: Xen, expert, ChenBeier

Post Reply
ferrouspatrick
Newbie
Newbie
Posts: 3
Joined: Wed Mar 30, 2022 7:57 am

Percentage concentration to molar concentration

Post by ferrouspatrick »

Hi there!

I'm having trouble calculating the molar concentration of 70% isopropanol (2-propanol). Said 70% is by weight. I'd really appreciate some help since i don't know how to calculate this.
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1542
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Percentage concentration to molar concentration

Post by ChenBeier »

Are you sure its by weight. Alcohol is normaly calculated by volume. Nevertheless weight means 1 kg contain 700 g Isopropanol = 70 % w.w.
If it is by volume its 700 ml in 1 l alcohol = 70% v.v.
To convert you need the specific gravity.
ferrouspatrick
Newbie
Newbie
Posts: 3
Joined: Wed Mar 30, 2022 7:57 am

Re: Percentage concentration to molar concentration

Post by ferrouspatrick »

Yeah, im sorry, i made a mistake. The actual concentration is 99.7% v-v, and the density is 0.786 g/cm3.
User avatar
ChenBeier
Distinguished Member
Distinguished Member
Posts: 1542
Joined: Wed Sep 27, 2017 7:25 am
Location: Berlin, Germany

Re: Percentage concentration to molar concentration

Post by ChenBeier »

So 1 l has a weight of 786 g, 99,7% v.v is 997 ml 100% alcohol. 3 ml = 3 g is water.. So the concentration is (786 g -3 g) / 0,997 l = 785,36 g/ l. The molar mass is 60 g/mol what gives a molarity of 13,08 mol/l.
ferrouspatrick
Newbie
Newbie
Posts: 3
Joined: Wed Mar 30, 2022 7:57 am

Re: Percentage concentration to molar concentration

Post by ferrouspatrick »

Thank you very much! That helps a lot.
Post Reply