Problem: An 8.65-g sample of an unknown group 2A metal hydroxide is dissolved in 85.0 mL of water. An acid-base indicator is added and the resulting solution is titrated with 2.50 M HCl (aq) solution. The indicator changes color signaling that the equivalence point has been reached after 56.9 mL of the hydrochloric acid solution has been added.What is the identity of the metal cation: Ca2+, Sr2+, Ba2+?

FREE Expert Solution

Recall that at the equivalence point of a titration:


moles acid=moles base


Also, recall that:

  • moles = molarity × volume
  • moles = molarity × molar mass


For acids: Multiply by the number of H+ ions

For bases: Multiply by the number of OH, H, O2–, or NH2 ions


This means:


MVacid=2(molesbase)


For Group 2A metal hydroxide → There are 2 OH 

  • M(OH)2


Solving for MMbase:


2.50 mol HClL(56.9 mL× 10-3 LmL)=2(8.65 gMM)

MM=2(8.65 g)0.14225 mol = 121.6 g/mol



Molar mass of metal hydroxide = molar mass of metal + 2(molar mass O) + 2(molar mass H)

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Problem Details

An 8.65-g sample of an unknown group 2A metal hydroxide is dissolved in 85.0 mL of water. An acid-base indicator is added and the resulting solution is titrated with 2.50 M HCl (aq) solution. The indicator changes color signaling that the equivalence point has been reached after 56.9 mL of the hydrochloric acid solution has been added.

What is the identity of the metal cation: Ca2+, Sr2+, Ba2+?

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