[vera]

[A] vs t plots for a zero-order is linear, and calculated reaction rate coefficient (k) has unit: mole L-1 sec-1.

If I have results for some component in concentration mg/L, ng/g, etc. and I plot concetration vs time (it is linear!), what will be the unit for reaction rate ? Or I have to calculate molar concentration from mass concentration always before ploting, in order to get reaction rate with correct unit ? 
Also, I have measured changes of mass concentration on the time scale in minutes, not in seconds ? Should I always use time scale in seconds ?

Thank you in advance for the answer !

[sondakem]

You answered your own question at the begining.  Mole/L/sec is a conc/time unit.  Just be consistent.  Covert your measured units to the units required or convert the contant to the units you want.  Be consistent.

Good luck

[vera]

Thank you very much for the answer.

So, it will not be mistake if I take reaction rate from graph mg/L versus time in minutes (this will be "mg/L/min") and at the end I just convert this reaction rate unit to "mole/L/sec" ?

According to this, it is not necessary to convert first all the data in mole/L, then plot the graph and calculate "k"?

[Yggdrasil]

As long a you plot time v. concentration, you should get a meaningful value of k.  However, when you calculate the rate constant, the units of k will depend on the units of your axes.  For example, if you plot seconds v. mol/L your calculated k will be in units of mol/L/s.  If you plot minutes v. mg/mL, then your calculated k will be in units of mg/mL/min (which can be easily converted back to mol/L/s if necessary).

[vera]

Thank you very much for the answer !