How to measure the amount of reactant used:
If one of the products is gas measure the mass before and after the experiment along with the time it takes for the experiment to be completed
The units for the rate if reaction can be given in g/s
The amount of reactant can be measured in moles
As the reaction takes place the reactant will be used up so the remaining amount of reactant will decrease
The concentration of the reactant is calculated by amount (mol) over volume (dm^3) measure is mol/dm^3