Practice worksheet

Question

Copper I iodide , Cul , is not stable enough to last long in storage , so it is generally made just prior to use . In can be prepared from copper sulphate and hydriodic acid by the following reaction . SHOW WORK ! 2CuSO4 +4 HI ---->

2 Cul+2H2SO4 +I2 If 34.56 grams of HI is used calculate the number of grams of each of the products that are produced

Answer 1

Hi Emily,

So I would first begin and say it's important to understand the relationship that links mass, molar mass and moles. I teach the formula MASS = Mr x MOLES (where Mr is the molar mass). I begin by writing the given mass under the compound (see below)

2 CuSO₄ + 4 HI ----> 2 Cul + 2 H₂SO₄ + I₂

34.56 g

So we first need to find the moles of the given compound, which is the HI. Use the equation above and rearrange to solve for moles: You need to be able to work out the molar mass of these compounds using the Periodic Table!

Moles = Mass = 34.56

Mr 127.911

= 0.27 moles HI

We will then use the balanced chemical equation to use those coefficients in front of the compounds to understand the mole ratio of different compounds. We will start by solving for the copper iodide.

Since HI and CuI are in a 4:2 ratio...you will take your moles of HI and divide by 2 to get moles of CuI.

Equation: 0.27 moles HI x 2 moles of CuI = 0.1351 moles of CuI

4 moles of HI

Now that you have moles of the product you're looking at...use the molar mass (Mr) to find that amount in grams. Equation: MASS = Mr x MOLE

Mass CuI = 190.45 x 0.1351 = 25.73 grams of CuI formed

You then repeat that same process to find the mass of the other 2 products

1) Use the mole ratio from your balanced equation to find the moles of the product

2) Change moles of product to mass of product using the relationship between mass, molar mass, and moles

Equation: 0.27 moles HI x 2 moles of H₂SO₄ = 0.1351 moles of H₂SO₄

4 moles of HI

Equation: 0.27 moles HI x 1 moles of I₂ = 0.0675 moles of I₂

4 moles of HI

Finally change both of those to mass using the molar mass

Mass H₂SO₄ = 98.079 x 0.1351 = 13.25 grams of H₂SO₄ formed

Mass I₂ = 253.8089 x 0.0675 = 17.13 grams of I₂ formed

Please NOTE: a lot of teachers teach a method where you do this all in one line and if you prefer that method it works exactly the same :)