 # Hints for February 26 Unit Factoring Worksheet

Here are the problems from today's worksheets with some hints to help you.  Quiz is tomorrow (or our next class period).  Remember - doing well on this quiz means you can also eliminate an old UF score!!!

1. (Easy) If every daisy has 29 petals, how many daisies will you need to have 400 petals?

Hint: One step problem.

1. (Easy, 2 part) How many cars can you make if you have 20,016 tires and 45,000 steering wheels?  Use unit factoring and convince me.  This will take two U.F. problems.

Hint: Use two separate problems (each is one step).  What is your limiting factor?  This is like the “how many pizzas can you make” problem.

1. (Medium-easy) How many moles of HCl are dissolved in 50 ml of a 2M solution of HCl?

Hint: “2M” means we have a “2 molar” solution.  This is a technical chemistry term that means there are 2 moles of HCl dissolved in enough water to make 1 liter of solution.  The conversion is 2 mol of HCl = 1 L of solution.  This problem is two steps.  (P.S. All molarity conversions have some number of moles of solute per liter of solution.)

1. (Medium) How many grams of zinc will react with 50 ml of a 2M solution of HCl?  Hint: You will have to write and balance the single displacement reaction to get mole ratios.

Hint: This starts exactly like problem #3.  You then have to take moles of HCl to moles of Zn using the mole ratios from the balanced equation.  Then do the molar mass of Zinc using the periodic table to finish the problem.

1. (Medium-advanced) If you react 50 grams of zinc with 50 grams of HCl, which will run out first?

This is also like the “how many pizzas” and “how many cars” problems in that you have to find out what you will run out of first –aka the limiting reagent. There are two ways to do this problem. For the first, start with 50 g of zinc. Use the periodic table to get molar mass, then use the mole ratio from the balanced equation. (It is the same equation used #3 and 4 – you’re welcome! ) After you have moles of HCl, convert that to grams, again using the periodic table.  If your answer is more than 50 g of HCl, that means to dissolve 50 g of zinc you need more than 50 g of HCl.  You only have 50 g, so you would not have enough HCl.  However, if your answer is less than 50 g of HCl, you have extra HCl, and thus your limiting factor is the zinc.

To answer the problem the other way, start with 50 g of HCl and go to grams of Zinc.Use the same logic. I recommend doing both to confirm your answer.

1. (Medium-advanced) I have made a solution of potassium chloride by dissolving 1 mole of potassium chloride into enough water to make one liter.  If I now take 250 milliliters from that solution, how many grams of potassium chloride will I have dissolved in the water?

Hint: This is a 1M solution of potassium chloride. (Don’t forget how to write potassium chloride correctly.)

Spoiler!Start with 250 ml of solution, take ml of solution to L of solution, L of solution to moles of potassium chloride (using the 1M info), and finally moles of potassium chloride to g of potassium chloride.

1. (Advanced) I have made a solution of potassium chloride by dissolving 1 mole of potassium chloride into enough water to make one liter.  If I now take 250 milliliters from that solution, how many moles of chloride ions will I have dissolved in the water?

Hint:  This starts out like question #6.  However, once at moles of potassium chloride, you will use the conversion that states how many moles of Cl- are in one mole of potassium chloride.

1. (Advanced, also requires equation)  How many grams of sodium hydroxide will be required to neutralize 100 ml of a 1M hydrogen chloride solution?

Hint: This is like the other molarity equations. Start, then take mL of solution to L of solution, L of solution to moles of solute. You will now need the balanced chemical equation (it is a double displacement) to get the molar ratio between hydrogen chloride and sodium hydroxide. Finally, take moles of sodium hydroxide to grams of sodium hydroxide. Remember, hydroxide is a polyatomic with a -1 charge (OH-).

1. (Advanced, also requires double displacement equation)  If 20 grams of copper (II) chloride are reacted with silver nitrate, how many grams of solid product are produced?

Hint:  This is old-school like the ones we have done previously.  Write the double displacement equation and balance it.  Identify the precipitate (insoluble solute).  Take grams of copper (II) chloride to moles of copper (II) chloride.  Then use molar ratio (from the equation) and finally molar mass to finish.

1. (Medium-advanced, review #5) If you react 50 grams of zinc with 50 grams of HCl, how many moles of zinc chloride can you get?

Hint:  Remember #5?  If you know the limiting factor, you only need to do one unit factoring problem – your start is the one you will run out of first.  (This is the chemistry version of only being able to make 5004 cars – plenty of leftover steering wheels, but only tires for that many.)