annedike22
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Hi. So we've been given these questions on Amount of Substance and no matter how many ways that I attempt to answer, I can't find anyway to properly understand the question or to find the theory behind it. Any help would be appreciated.


Here's the Question:
A Pure solid is thought to be calcium hydroxide. The solid can be identified from its relative formula mass.

The relative formula mass can be determined experimentally by reacting a measured mass of the pure solid with an excess of HCl. The equation for this reaction is

Ca(OH)2 + 2HCl —> CaCl2 + 2H2O

The unreacted acid can then be determined by titration with a standard sodium hydroxide solution.

You are provided with 50.0 cm3 of 0.200 mol/dm3 HCl.

Outline, giving brief practical details, how you would conduct an experiment to calculate accurately the relative formula mass of the solid using this method.
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username5006362
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(Original post by annedike22)
Hi. So we've been given these questions on Amount of Substance and no matter how many ways that I attempt to answer, I can't find anyway to properly understand the question or to find the theory behind it. Any help would be appreciated.


Here's the Question:
A Pure solid is thought to be calcium hydroxide. The solid can be identified from its relative formula mass.

The relative formula mass can be determined experimentally by reacting a measured mass of the pure solid with an excess of HCl. The equation for this reaction is

Ca(OH)2 + 2HCl —> CaCl2 + 2H2O

The unreacted acid can then be determined by titration with a standard sodium hydroxide solution.

You are provided with 50.0 cm3 of 0.200 mol/dm3 HCl.

Outline, giving brief practical details, how you would conduct an experiment to calculate accurately the relative formula mass of the solid using this method.
Titrate the unreacted acid with a standard solution (a solution whose conc is known) of NaOH. Determine the volume of NaOH required to neutralise the unreacted acid. It's a 1:1 reaction, so the moles of unreacted acid and hence volume can be determined by calculating the moles of NaOH required to neutralise the unreacted acid.

Subtract the volume of the unreacted acid from the initial (total) volume of HCl, and you'll have the volume that reacted with the Ca(OH)2 mass, use it to calculate the moles and hence use the 1:2 ratio from the equation to calculate the moles of Ca(OH)2 that reacted with HCl.

You are already provided with the mass of Ca(OH)2 that reacts with HCl in excess. Once you have obtained the no of moles, you can use the equation no. of moles=mass÷mr to calculate the relative formula mass of Ca(OH)2.
Last edited by username5006362; 1 year ago
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