Chromium and its compounds
Answers are for parents and teachers.
1
Write equations for the changes Cr → CrCl₃ → Cr(OH)₃ → Cr₂O₃ (step 1: with Cl₂).
2Cr + 3Cl₂ → 2CrCl₃; CrCl₃ + 3NaOH → Cr(OH)₃↓ + 3NaCl; 2Cr(OH)₃ → Cr₂O₃ + 3H₂O.
2
How many grams of Cr₂O₃ form when 126 g of (NH₄)₂Cr₂O₇ decomposes? ((NH₄)₂Cr₂O₇ → Cr₂O₃ + N₂ + 4H₂O; M = 252 and 152)
76
3
Find the oxidation state of chromium in Na₂CrO₄ and explain whether it is an oxidising or reducing agent.
+6; this is chromium’s highest oxidation state, so it can only accept electrons — an oxidising agent.
4
How many moles of Cr atoms are in 1 mol of Cr₂O₃ and what is their mass? (A(Cr) = 52)
2 mol, 104 g
5
Why are precautions necessary when working with chromium(VI) compounds?
They are poisonous and harm skin and health; only the teacher works with them, using protection.