Respuesta :
0.784 g of the hydrochloride must be dissolved in 10.00 mL of water to give a pH of 3.50.
Calculation of mass of hydrochloride:
dissociation of thiamine hydrochloride:
C₁₂H₁₈ON₄SCl₂ ⇄ [tex]C_{12} H_{17} ON_{4} SCl_{2}[/tex]
HA + [tex]H_{2} O[/tex]⇄[tex]H_{3} O^{+} +A^{-}[/tex]
Henderson-Hasselbalch equation:
pH = pKa + log[tex][A^{-} ]/[HA][/tex]
log[tex][A^{-} ]/[HA][/tex] = 3.50 - [-log(3.37[tex]x10^{-7}[/tex])]
=3.50 - 6.47= -2.97
[tex][A^{-} ]/[HA][/tex] = [tex]10^{-2.97}[/tex]
= 1.07[tex]x10^{-3}[/tex][HA]
[tex][H^{+}][/tex] = [tex][A^{-} ][/tex] =[tex]10^{-pH} =10^{3.50} =3.16x10^{-4}[/tex]
[HA] = [tex]\frac{3.16x10^{-4} }{1.07x10^{-3} } =0.296M[/tex]
[tex]m_{HA}[/tex] = 0.01000L x 0.296 mol/L x 265.35g/1 mol
= 0.784 g
Hence, 0.784 g of the hydrochloride must be dissolved in 10.00 mL of water to give a pH of 3.50.
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