Question

the value of 4200 j kg-1 k-1 in standard form

Answer 1

In standart form this value will be written as:

$$4200J/(kg\cdot K) = 4.2\times 10^3J/(kg\cdot K)$$

or

$$4.2\space kJ/(kg\cdot K)$$

Answer. $4.2\times 10^3J/(kg\cdot K)$ or $4.2\space kJ/(kg\cdot K)$.