HardChemistryHigh School (11–12)Coordination Compounds › Werner's Theory, Ligands and NomenclatureNumericalA-LevelBoard ExamIB
A compound has the formula CrCl₃· nNH₃. Its molar mass is 243.5 g/mol…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A compound has the formula . Its molar mass is . Take Cr as , Cl as and as . The coordination number of chromium is 6. A sample of is dissolved in water, and excess silver nitrate gives of silver chloride, with . Find and the number of free chloride ions in one formula unit. Write the formula of the complex salt.
Need a reminder? The key ideaShow
Werner's theory: the primary valence is the oxidation state, and the secondary valence is the coordination number. The coordination sphere is the metal with its ligands, in square brackets. Only the counter ions outside the brackets are free in water, so only they give a precipitate with silver nitrate. A double salt gives all its ions. A complex keeps its complex ion.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. The molar mass gives . The silver chloride tells us how many chloride ions are free.
Step 1. The ammonia. has a molar mass of . The rest is ammonia: . So .
Step 2. The salt. .
Step 3. The silver chloride. .
Step 4. Free chloride in one unit. .
Step 5. The formula. Five ammonia molecules fill five of the six places. One chloride ion fills the sixth. The other two chloride ions are free. The formula is . It gives one complex ion and two free chloride ions, so 3 ions.
Answer: , 2 free chloride ions, 3 ions. The salt is .
Check. Six places: . Three chloride ions in all: .
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