OCR AS level titration

  • 1 vote

I have been given 25cm3 of 0.3moldm-3 concentration HCl that's been diluted to 250cm3 and a solution X(OH)2 (group 2 metal) that took 18.60cm3 to neutralise the acid. I need to work out the number of moles of the acid and the X(OH)2, and work out the concentration of the X(OH)2. It then says 'use the concentration and the fact that there are 1.110g of X(OH)2 in 1000cm3 to calculate the molar mass of the unknown hydroxide' then work out the identity of X from the periodic table. Please help someone!

Posted Sat 16th February, 2013 @ 20:06 by Joanne

1 Answer

  • 1 vote

rst find the number of moles of HCl by doing (250*0.3)/1000. write up the equation HCl + x(OH)2 = XCl2 + H2O, balance it to get 2HCl + X(OH)2 = XCl2 + 2H2O. so now what you got as your moles of HCl divide by 2 because the stoichiometry (basically the numbers infront of each molecule in an equation) shows that for every 2 moles of HCl there is one of X(OH)2... so now you have the number of moles of the unknown and you know you needed 18.6ml to neutralise, you can work out the concerntration using the same formula to find moles and just rearrange. C = (moles*1000)/volume so that gives C=(moles*1000)/18.6 (im not working out the moles because it will help you understand it better if you actually do it yourself i think than just copy) so now you have the moles of both the compounds and the conc of the unknown compound. It then gives you the mass and a volume. so using the concerntration youve just worked out and the volume they give (1000) you can work out the moles of this different solution. moles = (conc you found * 1000)/1000 ( you might be able to see the volume and the 1000 you need to convert it to dm3 will cancel so the moles of the solution will equal the conc, but dont worry if thats confusing at all), now you have th moles of the solution and all you need to do is rearrange the equation for finding moles in a solid moles=mass/molecular weight so you re arrange to give the equation molecular weight(molarmass) = mass/moles then just use the periodic table to find a group 2 metal with the closest number. (remember to take of the 2 OH molecules first to get the weight of the X) but I have tried working this out myself and I think on of the numbers might be wrong because it doesnt give you an appropriate molecular weight... hope the method helps though

Answered Mon 4th March, 2013 @ 13:00 by Briony Crush