Sodium hydroxide, also known as lye or caustic soda, is a highly corrosive substance that can cause severe burns and damage to the skin and eyes. Weighing sodium hydroxide on a piece of filter paper would be dangerous and could result in the substance coming into contact with the skin or eyes. Weighing sodium hydroxide in a bottle allows for safe and accurate measurement of the substance while minimizing the risk of accidents or spills. Here are a few reasons why sodium hydroxide is typically weighed in a bottle:

Safety: Sodium hydroxide can react with moisture in the air to produce a caustic mist, which can be harmful if inhaled. Weighing sodium hydroxide in a closed bottle helps prevent the release of this mist and reduces the risk of exposure.

Precision: Weighing sodium hydroxide in a bottle allows for more precise measurements compared to using a piece of filter paper. The bottle provides a stable and controlled environment for weighing, minimizing the effects of air currents and other environmental factors that could affect the accuracy of the measurement.

Convenience: Weighing sodium hydroxide in a bottle is more convenient and reduces the risk of spills or contamination. The bottle can be easily sealed and stored, preventing the substance from coming into contact with other materials or the environment.

Stability: Sodium hydroxide can absorb moisture from the air, which can affect its weight and reactivity. Weighing sodium hydroxide in a closed bottle helps maintain its stability and prevent changes in its composition due to exposure to moisture.

Therefore, weighing sodium hydroxide in a bottle is the preferred method for safety, precision, convenience, and stability.