A pipe closed at one end has length $0.8 \mathrm{~m}. At its open end 0.5 \mathrm{~m} long uniform string is vibrating in its 2^{\text {nd }} harmonic and it resonates with the fundamental frequency of the pipe. If the tension in the wire is 50 \mathrm{~N} and the speed of sound is 320 \mathrm{~m} / \mathrm{s}$, the mass of the string is