Answer to Question #305841 in General Chemistry for dadw

Question

When calculating the mass of Cu2Se formed when 40.0g copper metal reacts with 28.0g selenium. What is the amount of excess reactant that is left over.

Answer 1

• Step 1: Write a balanced equation
• 2Cu + Se ----> Cu₂Se
• Step 2: Find the moles of copper and selenium in their masses.
• 64g of copper is in 1 mole, what about 40g
• 40/64 = 0.625moles of copper.
• 79g of selenium is in 1 mole, what about 28g
• 28/70 = 0.4 moles of selenium.
• Finally, find the excess reactant by using mole ratio.
• Mole ratio copper: selenium is 2:1
• 0.625/2=0.3125 moles of copper vs 0.4 moles of selenium.
• You realize that selenium is in excess by 0.0875 moles
• 0.4-0.3125 = 0.0875 moles.
• This can also be converted into mass by multiplying by 79g to get 6.9125grams of selenium.