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Topic: Please help Mass percent problem  (Read 4880 times)

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Please help Mass percent problem
« on: September 25, 2004, 01:26:28 PM »
hello i am trying to do this problem as one of my chemistry online quizzes. the problem is :

Morton Lite SaltĀ® has 290 mg of sodium (as Na+) and 340 mg of potassium (as K+) per 0.25 teaspoon. Assume that 1.0 teaspoon of the Lite Salt has a mass of 6.0 g and that the Na+ comes from NaCl and the K+ from KCl. Calculate the mass percent of NaCl and of KCl in Lite Salt.

I tried working it out and i got 48% and 52% respectively but it's saying that its the wrong answer. please help. THNAK YOU


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Re:Please help Mass percent problem
« Reply #1 on: September 25, 2004, 04:55:49 PM »
You are assuming that Na,  K and Cl are the only things in the salt. Not necessarily so, meaning the sum of the NaCl and KCl  could be less than 100%.
Calculate each % mass separately using the 6.0 g as the total mass in the denominator.

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Re:Please help Mass percent problem
« Reply #2 on: September 28, 2004, 04:01:52 PM »
Here's how I would handle it.  First off, you're given the masses in mg of the sodium and the potassium present in 0.25 tsp.  Then they give you the mass of 1.0 tsp in grams (6.0 grams).  So I'd convert the mg into grams.  This will give you 0.290 grams of Na and 0.340 grams of potassium.  Those masses will be present in 6.0/4= 1.5 grams of salt.  Now, you need to find the number of moles of sodium and potassium you have present since each mole of sodium and each mole of potassium have one mole of chlorine.  So for sodium you would find the number of moles you have, then multiply by the molar mass of chlorine to get the number of moles of chlorine from the NaCl.  Then you would do the same for the potassium.  Now for each of the salts just add together the mass of the metal and the chlorine, and divide by the mass of 0.25 tsp of salt (1.5 grams).  Multiply by 100 and there is your percentage for each of them.  You will find that the % will NOT equal 100% because Iodide ions are added to salt in very small amounts.  Does that make sense?
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