Need help with molarity problems

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1. If I had a 1M solution of sucrose, how would I make a 0.8M solution of sucrose?

2. How would I make 100ml of a 2M solution of sucrose?

3. How would I make a 10% solution of glucose?

4. How can I make 500 ml of a 10% solution of glucose?

Aug 27th, 2015

Hi there! Thank you for the opportunity to help you with your question!

The definition of molarity is: Moles of solute/liters of solution. Therefore:

1. You have a 1M solution of sucrose, that is 1 mole/1 liter.  In order to make a 0.8M solution of sucrose you have to add some liters of solution, call that number x.

So 1 mole/(1+x) liters has to be equal to 0.8, meaning that 1 mole/0.8 = (1+x) liters, x = 0.25. This means you have to add 0.25L, that is 250mL, of solution

2. To make 2M solution of sucrose with 100mL (or 0.100L) of solution, you would have to have x moles of sucrose/0.100L of solution = 2M. This means that x= 0.2 moles. Now, to get the amount of sucrose in grams you have to find how many grams per mole of sucrose: The mass of 1 mole of sucrose = (12 * 12) + (22 * 1) +(11 * 16) = 342 grams/mole. Since we only need 0.2 moles, we need 342*0.2 = 68.4 grams of sucrose in 100mL of solution

3. Now we are talking about percentage solution; we need the percent weight/volume of glucose to be 10%. This means we need 100g of glucose for every liter of solution.

4. We need (mass of glucose)/500 mL = 10%, therefore mass of glucose = 50 grams

Please let me know if you need any clarification. Always glad to help!
Aug 27th, 2015

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