A source of sound produces a note of 512 Hz in air at 17 degree celsius with wavelength 66.5 cm. Find the ratio of molar heat capacities at constant pressure to constant volume at NTP. Densities of air and mercury at NTP are 1.293 kg/m^3 and 13600 kg/m^3 respectively.

a. 0.32 b. 1448 m/s

Explanation:

We know v ∝ √T where v = velocity of sound and T = absolute temperature.

Let v₁ = velocity of sound at 17°, v₁ = fλ where f = frequency of sound = 512 Hz and λ = 66.5 cm = 0.665 m

So, v₁ = fλ = 512 Hz × 0.665 m = 340.48 m/s

T₁ = 17 + 273 = 290 K

Let v₂ = velocity of sound in air at NTP = unknown and T₂ = temperature at NTP = 0°C + 273 = 273 K

Now v₁/v₂ = √T₁/√T₂

So, v₂ = (√T₂/√T₁)v₁

= [√(T₂/T₁)]v₁

substituting the values of the variables, we have

v₂ =  [√(273 K/290 K)]340.48 m/s

v₂ =  [√0.9413]340.48 m/s

v₂ = (0.9702)340.48 m/s

v₂ = 330.35 m/s

Also v = √(γP/ρ) where v = velocity of sound in air at NTP  = 330.35 m/s, γ = ratio of molar heat capacities, P = pressure at NTP = 1.013 × 10⁵ Pa and ρ = density of air = 1.293 kg/m³

Since,  v = √(γP/ρ)

making γ subject of the formula, we have

γ = v²ρ/P

substituting the values of the variables, we have

γ = (330.35 m/s)² × 1.293 kg/m³/1.013 × 10⁵ Pa

= 31975.36 kg/m²s² ÷ 1.013 × 10⁵ Pa

= 0.32

b. Speed of sound in mercury v₃ = √(B/ρ) where B = Bulk modulus of mercury = 28.5 × 10⁹ Pa and ρ = density of mercury = 13600 kg/m³

v₃ = √(B/ρ)

= √(28.5 × 10⁹ Pa/13600 kg/m³)

= √(28.5 × 10⁹ Pa/13.6 × 10³ kg/m³)

= √(2.096 × 10⁶) m/s

= 1.448 × 10³ m/s

= 1448 m/s

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,

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