HELPPP After 25.3 seconds, the concentration of a starting material
was measured to decrease from 2.00 M to 1.17 M.

Rate = 0.83M/s

Explanation:

Rate of a chemical reaction is the 'absolute value' of change in concentration per unit time. That is ...

Rate = |Change in concentration / unit time|

= |Δ[Conc]/Δtime|

= |(1.17M -2.00M) / 25.3s|

= |- 0.83M/s|

= 0.83M/s*

*Note: Some scholars define rate of reaction as a 'scaler' quantity having only magnitude as rate of reaction in a physical sense (up, down, right, left) is not definitive. In physical mechanics, rate is assigned a direction and magnitude of action and becomes a 'vector' quantity.