Molar dilutions

[oncetoomany]

Ok got a problem.

There is a 140mmol/L stock standard of Na and the working standard made from this is 70mmol/L. If i follow the instructions i get 6.4mmol/L of Na.

 

Instructions:

Mix:

0.50mL Li standard

0.025mL Na stock standard (140mmol/L)

4.475mL water

 

If i mix these my calc. shows it'll be 6.4mmol/l

calculations:

140mmol/L / 1000 = 0.14mmol/mL <-L to mL

0.14mmol/mL * 0.025mL = 0.0035mmol/0.025mL <- how many moles are in 0.025mL

0.0035mmol/5mL <-becasue there is five mL total in the mix

0.035mmol/5mL * 200 = 6.4mmol/L <-converting 5mL into 1L

 

please can someone help me find out what im doing wrong? please post with the working out.

[Hiya]

I'm not sure how you managed to get 6.4mmol per litre, but

 

0.0035mmol/5mL

 

Ok so is 0.0035mmol in 5ml, meaning that in every ml there is 0.0007mmol or 0.7mmol/L

 

From the top..

 

0.000025L Na solution * 140mmol/L = 0.0035mmol Na

 

0.0035mmol Na divided between a 0.005L solution = 0.7mmol/L

 

Not sure where 70mmol/L comes from, but it's 3 orders of magnitude bigger so I imagine there is some inaccuracy in your write-out between mmol and mol or L and ml

[Dave]

Moving this to the Chemistry forum, don't think it belongs here

[budullewraagh]

sorry, but may i ask what solvent you are using?