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It is to accommodate for the sign of the change in volume. If work is being done on the system (meaning work is positive), the volume of the system will decrease (ex: piston being pushed in) and the change in volume will be negative. Without the negative in front of the equation to counterbalance the negative change in volume, you would obtain a negative value for work done instead of a positive one. Hope this helps!
When work is done by the system, the system will lose energy to its surroundings. The negative sign in the equation w = - (P * delta V) accounts for this loss of energy by the system when it performs work. Negative work is performed by the system when the volume increases or expands at constant pressure. Positive work is performed by the system when the volume is compressed at constant pressure.
When it is being compressed and volume is decreasing then work done on the gas/system is positive and gas done by the system is positive. When there is an expansion there is negative work done on the gas and positive work done by the gas.
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