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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: dirknathan on November 27, 2012, 03:27:44 PM

Title: This molarity problem looks so simple!
Post by: dirknathan on November 27, 2012, 03:27:44 PM: Yet, I can't see the silver lining.

In a routine analysis a solution of sodium chromate with a chromium concentration of 75.0 ppm is required. Calculate the molarity of the solution.

Now, does ppm equal mg/L?

Thanks for the *delete me*
Title: Re: This molarity problem looks so simple!
Post by: Borek on November 27, 2012, 03:30:18 PM: Quote from: dirknathan on November 27, 2012, 03:27:44 PM
Now, does ppm equal mg/L?

Usually yes.
Title: Re: This molarity problem looks so simple!
Post by: dirknathan on November 27, 2012, 04:36:49 PM: Well, is it relevant to this problem?
Title: Re: This molarity problem looks so simple!
Post by: Hunter2 on November 27, 2012, 05:05:44 PM: Yes it is.
Title: Re: This molarity problem looks so simple!
Post by: dirknathan on November 27, 2012, 06:09:00 PM: How do I relate the Sodium to that of mg/L?
Title: Re: This molarity problem looks so simple!
Post by: Borek on November 27, 2012, 06:35:11 PM: Through the molar masses. What molar concentration of sodium chromate has 75 mg Cr per liter?
Title: Re: This molarity problem looks so simple!
Post by: dirknathan on November 28, 2012, 12:28:21 AM: I've figured out the molarity of Cr. how does one find the molarity of the entire solution with the molarity of one part?
Title: Re: This molarity problem looks so simple!
Post by: Hunter2 on November 28, 2012, 12:42:15 AM: Mole is the base. How many Chromium atoms you have in one molecule Chromate? If you know the mole of Chromium then you also know it from the Chromate then.
Title: Re: This molarity problem looks so simple!
Post by: dirknathan on November 28, 2012, 11:51:01 AM: What about the oxygena in Chromate?
Title: Re: This molarity problem looks so simple!
Post by: Borek on November 28, 2012, 01:40:30 PM: Does the presence of oxygen change the concentration of chromium?
Title: Re: This molarity problem looks so simple!
Post by: dirknathan on November 28, 2012, 01:54:14 PM: Chromate*
Title: Re: This molarity problem looks so simple!
Post by: Hunter2 on November 28, 2012, 01:58:35 PM: What is the formula of sodium chromate.
If you know this 1 mole of it containd how many mole sodium, chromium and oxygen.
Title: Re: This molarity problem looks so simple!
Post by: dirknathan on November 28, 2012, 02:28:18 PM: Na₂CrO₄
Title: Re: This molarity problem looks so simple!
Post by: Hunter2 on November 28, 2012, 02:31:08 PM: Great.
If you have 1 mol of sodium chromate, how many moles of sodium it contains, the same for chromium and oxygen.
Title: Re: This molarity problem looks so simple!
Post by: dirknathan on November 28, 2012, 02:52:10 PM: So two moles of Na, 1 of Cr and 4 of O? Now do they all have 75 ppm?
Title: Re: This molarity problem looks so simple!
Post by: Hunter2 on November 28, 2012, 03:45:52 PM: So you learned that 1 Cromium correspond to 1 Sodiumchromate

1 Cr ==> 1 Na₂CrO₄

Now calculate how much moles 75 ppm Cr is.

If you know this calculate back how much Sodiumchromate correspond to it.
Title: Re: This molarity problem looks so simple!
Post by: Borek on November 28, 2012, 04:40:38 PM: Quote from: dirknathan on November 28, 2012, 02:52:10 PM
So two moles of Na, 1 of Cr and 4 of O? Now do they all have 75 ppm?

Not all, just Cr.

And the question I asked earlier was about chromium, not chromate. Does the presence of oxygen change the concentration of chromium? Once you will understand the question and the answer, you will see what it is all about.