Molarity of acids?
a) 0.5 NH3PO4 ?
b) 0.5 NH2SO3 ?
Answer:
As is clear from its name, molarity involves moles. Boy, does it!
The molarity of a solution is calculated by taking the moles of solute and dividing by the liters of solution.
This is probably easiest to explain with examples.
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Example #1 - Suppose we had 1.00 mole of sucrose (it's about 342.3 grams) and proceeded to mix it into some water. It would dissolve and make sugar water. We keep adding water, dissolving and stirring until all the solid was gone. We then made sure that when everything was well-mixed, there was exactly 1.00 liter of solution.
What would be the molarity of this solution?
The answer is 1.00 mol/L. Notice that both the units of mol and L remain. Neither cancels.
A replacement for mol/L is often used. It is a capital M. So if you write 1.00 M for the answer, then that is correct.
Some textbooks make the M using italics and some put in a dash, like this: 1.00-M. When you handwrite it; a good, old block capital M is just fine.
When you say it out loud, say this: "one point oh oh molar." You don't have to say the dash.
And never forget this: replace the M with mol/L when you do calculations. The M is just shorthand for mol/L.
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Example #2 - Suppose you had 2.00 moles of solute dissolved into 1.00 L of solution. What's the molarity?
The answer is 2.00 M.
Notice that no mention of a specific substance is mentioned at all. The molarity would be the same. It doesn't matter if it is sucrose, sodium chloride or any other substance. One mole of anything contains 6.022 x 1023 units.
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Example #3 - What is the molarity when 0.75 mol is dissolved in 2.50 L of solution?
The answer is 0.300 M.
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Now, let's change from using moles to grams. This is much more common. After all, chemists use balances to weigh things and balances give grams, NOT moles.
Example #4 - Suppose you had 58.44 grams of NaCl and you dissolved it in exactly 2.00 L of solution. What would be the molarity of the solution?
The solution to this problem involves two steps which will eventually be merged into one equation.
Step One: convert grams to moles.
Step Two: divide moles by liters to get molarity.
In the above problem, 58.44 grams/mol is the molecular weight of NaCl. (For you technical types, I know it actually is a formula weight, but I'm glossing over the difference for the time being. Remember, this is a high school tutorial.)
Dividing 58.44 grams by 58.44 grams/mol gives 1.00 mol.
Then, dividing 1.00 mol by 2.00 L gives 0.500 mol/L (or 0.500 M). Sometimes, a book will write out the word "molar," as in 0.500-molar.
a) 0.5 N H3PO4 has 3 equivalents of H+
so it's 1/3*(0.5) = 0.17 M H3PO4
b) 0.5 N H2SO3 has 2 equivalents of H+
so it's 1/2*(0.5) = 0.25 M H2SO3
Molarity - weren't they 2 of the 3 Stooges?
normal is a term that is directly related to molarity in that molarity refers to the concentration of molecules you have in a solution and normality refers to the concentration of protons those said chemicals have that are acidic. for example, 1 mol/L (M) of Hydrogen Chloride (HCl) is 1N. 1M of Sulfuric Acid (H2SO4) is 2N. 1M of Phosphoric Acid (H3PO4) is 3N.
Remember, this only applies to acidic protons (hydrogens), but all of the hydrogens in your problem are considered to be acidic.
you should be able to figure it out from there ;)
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