Suppose you need to determine the vapor pressure of water at 25.4°C.

You know that at 24.0°C the vapor pressure is 2.985 kPa and at 25.0°C it is 3.169 kPa.

First, determine the difference between the two pressures: 3.169 - 2.985 = 0.184 kPa.

Second, multiply the difference by the decimal portion (ONLY!) of the desired temperature: 0.184 x 0.4 = 0.0736 kPa.

Third, add this value to the LOWER pressure you used in the first step: 2.985 + 0.0736 = 3.0586 kPa.

Last, round off to the appropriate number of significant figures. In this case, the answer would be 3.059 kPa.

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