Final answer:
Using a proportion based on heat capacity, increasing the amount of water from 50.0 mL to 116.3 mL results in approximately a 2.326 times increase in mass, thereby reducing the temperature change to about 4.3 ºC, assuming the reaction's change in enthalpy stays constant.
Step-by-step explanation:
If a reaction causes 50.0 mL of water to increase temperature by 10.0 ºC at constant pressure, then using the concept of heat capacity, we can understand that the amount of heat needed to raise the temperature of water is proportional to its mass. When you increase the amount of water, the same amount of heat will be distributed over a larger mass, resulting in a smaller temperature increase.
Given that the change in enthalpy of the reaction remains the same, if we use 116.3 mL of water instead of 50.0 mL, the mass of the water is increased by a factor of 116.3/50 or approximately 2.326 times. This would mean that to achieve the same change in enthalpy, the temperature increase would be divided by this factor. If we divide the original temperature change of 10.0 ºC by 2.326, we would find the new temperature change.
Therefore, the new temperature change would be approximately 4.3 ºC (rounded to one decimal place).