Preparing Solutions and Molarity

  • Thread starter azncocoluver
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    Molarity
In summary: For #2, I would need to find the density of the 5% NaCl solution and then use the molarity and mass to calculate the amount of NaCl.
  • #1
azncocoluver
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Homework Statement


Calculate how many grams of solute are necessary to prepare:
1) 100 mL of 0.1 N (Normal Solution) Ca(OH)2 (molecular weight = 74)

2) 100 mL 5% NaCl (molecular weight = 40)


Homework Equations


Molarity = # of moles / # of liters


The Attempt at a Solution


1. I set up dimensional analysis.

74g Ca(OH)2 divided by 2 since there are 2 Hydrogens = 37g Ca(OH)2 for Molecular Weight

100ml * (1 L / 1,000 mL) * (.05 mol / 1 L) * (37g / 1 mol) = 0.185g

I'm not sure if it's really .05 mol/L for Ca(OH)2. I just figured since the molarity was 0.1 N that I could divide 0.1 by 2 hydrogens to give me .05 mol/L

2. I honestly do not know how to even start this..
 
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  • #2
1.) Prepare 0.2 M solution of Calcium Hydroxide.

2.) Just take 5 gms of NaCl, if we are talking about % w/v, which generally is the case.
 
  • #3
Why is it 0.2M for #1? And can you show me how to set up for #2 please? Thank you once again!
 
  • #4
Not 0.2, but 0.05 M Ca(OH)2. 0.05M solution will be 0.05M in Ca2+ and 0.1M in OH- - compare with the definition of normality.

It is not clear if you are to prepare 5% w/w or w/v solution (not that the difference will be large). If w/v - by definition of the w/v it means 5 g in 100 mL of the solution. If w/w - approach depends on how accurate you want to be.

Simple and less accurate approach - dilute solutions have density of 1 g/mL. That means you will need 100 g of solution. Of this 5% is NaCl and 95% is solvent (w/w). Use this information to calculate mass of NaCl.

Exact approach (but most likely overkill) is to check the density of the solution in the density tables (for 5% NaCl it is 1.034 g/mL), calculate mass of the 100 mL - and do exactly the same thing you did in the above case.
 
  • #5
So for #1 would I set it up like this:

# moles of Ca(OH)2 = 0.100 ml * {0.100 moles of OH(!)/l} * (1 mole of Ca(OH)2 / 2moles of OH)
 
  • #6
0.100 L, not 0.100 mL. Other than that, looks OK to me.
 

1. What is the definition of "molarity"?

Molarity is a unit of concentration that measures the number of moles of a substance dissolved in a liter of solution. It is commonly denoted by the symbol "M" and is expressed in units of moles per liter (mol/L).

2. How do you prepare a solution with a specific molarity?

To prepare a solution with a specific molarity, you will need to know the desired molarity, the volume of the solution you want to make, and the molar mass of the solute. You can then use the formula M = moles of solute / volume of solution (in liters) to calculate the amount of solute needed.

3. Can I use any unit of measurement for molarity?

No, molarity must be expressed in units of moles per liter (mol/L). Using any other unit of measurement would result in an incorrect molarity value.

4. How do I convert between molarity and other units of concentration?

To convert between molarity and other units of concentration, you will need to know the molar mass of the solute and the volume of the solution. For example, to convert from molarity to molality (moles of solute per kilogram of solvent), you can use the formula molality = molarity * molar mass / density of solvent.

5. What are some common mistakes to avoid when preparing solutions and calculating molarity?

Some common mistakes to avoid include using the wrong units of measurement for molarity, not properly accounting for the volume of the solute when calculating the amount needed, and not accurately measuring the volume of the solution. It is also important to properly label solutions and to always double-check your calculations.

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