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.

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

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

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