solution titration help

[saintxiao]

1.A 78.6-mL sample of HCl(aq) is diluted to a volume of 500.0 mL. If the concentration of the diluted solution is found to be 0.393 M HCl, what was the concentration of the original solution?

 

0.0786 L x 0.393 M = 0.500 L X ?M

 

solution is 0.0618 M HCL

 

is that right?

 

2.Assuming the volumes are additive, what is the [NO3-] in a solution obtained by mixing 174 mL of 2.50 M KNO3, 218 mL of 0.750 M Mg(NO3)2, and 145 mL of H2O?

anyone show me how to do this question?

[Phi for All]

I thread per question please, I removed the duplicate thread in Homework Help.

[SciGuyCT]

Saint,

 

1. Make sure that you are keeping track of which side of the equation your numbers should be on. Also, check to make sure that your answer makes sense. If the diluted solution has a concentration of 0.393M HCl then how can the undiluted solution be less concentrated (0.0618 < 0.393).

 

2. First, look at the first part of the sentence. *Assuming the volumes are additive* What should be the first step to solving this problem? . Next, you have molar concentrations and volumes of several different compounds. You should attempt to figure out how much NO3- comes from each compound (You should be able to do this ) The easy part of this equation is that the NO3- is completely dissociative so you don't have to worry about how much free NO3- there is compared to bound. Once you have the amount from each compund you should be able to add them up and figure out the total concentration. Don't forget about the water!!!

 

Let me know if you need any more help

[saintxiao]

Saint,

 

1. Make sure that you are keeping track of which side of the equation your numbers should be on. Also, check to make sure that your answer makes sense. If the diluted solution has a concentration of 0.393M HCl then how can the undiluted solution be less concentrated (0.0618 < 0.393).

 

2. First, look at the first part of the sentence. *Assuming the volumes are additive* What should be the first step to solving this problem? . Next, you have molar concentrations and volumes of several different compounds. You should attempt to figure out how much NO3- comes from each compound (You should be able to do this ) The easy part of this equation is that the NO3- is completely dissociative so you don't have to worry about how much free NO3- there is compared to bound. Once you have the amount from each compund you should be able to add them up and figure out the total concentration. Don't forget about the water!!!

 

Let me know if you need any more help

 

 

thanks for help but i still have problem with second question.

 

i use

 

174ml kno3 of 2.50 M kno3 find out 0.435 mol No3

 

and use

 

218 ml mg(No3)2 of 0.750 M mg(No3)2 find out 0.3256 mol No3

 

 

what should i do next?

[SciGuyCT]

Well you have pretty much solved the problem on your own. You now have the number of moles of NO3-. All you need to do to solve the problem is find out the concentration (moles per volume). Make sure to take into account ALL the components (*cough* water *cough*).

[hermanntrude]

generally I find the best way to approach a titration problem (unless it's one designed to be confusing) is a three step process

 

1) find the moles of titrant using c=n/v

2) do some stoichiometry based on the equation (figure the equation out first if you have to)

3) find the moles or concentration of your original sample using c=n/v

 

sometimes there are additional steps like finding the mass of the original sample or finding the purity of a sample you dissolved to titrate or something like that.