Given question is:
Given a food with a relative humidity of 81%, what is the water activity in that food?
Step-by-step explanation:
Water activity in food can be determined by using the below-shown formula:
![water activity=(equilibrium relative humidity)/(100)](https://img.qammunity.org/2022/formulas/chemistry/college/gcjwdkwu1gpc425t5m9m06oa0fm1myoas7.png)
Equilibrium is established between the vapor pressure of food and the surrounding air media.
Thus, for the given food the relative humidity is 81%.
hence, its water activity is
![81/100\\=0.81](https://img.qammunity.org/2022/formulas/chemistry/college/jgnhss7jo9wp2b1as1w7guhmp2qq11dglw.png)