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Polyprotic Buffers quiz

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  • What is typically meant by a polyprotic buffer in analytical chemistry?
    A polyprotic buffer usually refers to a triprotic buffer, which contains an acid with three acidic hydrogens.
  • How many Ka values does a triprotic acid have?
    A triprotic acid has three Ka values, one for each acidic hydrogen that can be removed.
  • What is the fully protonated form of a triprotic acid commonly represented as?
    The fully protonated form is represented as H3A.
  • What is the first intermediate species formed after removing one hydrogen from a triprotic acid?
    The first intermediate is H2A⁻, formed after removing the first acidic hydrogen.
  • Which Ka value is used when relating the fully protonated form and the first intermediate in the Henderson-Hasselbalch equation?
    Ka1 is used when relating H3A and H2A⁻ in the Henderson-Hasselbalch equation.
  • In the Henderson-Hasselbalch equation for polyprotic buffers, which form goes in the numerator?
    The more basic form, which has one less acidic hydrogen, goes in the numerator.
  • What is the second intermediate species in the dissociation of a triprotic acid?
    The second intermediate is HA²⁻, formed after removing the second acidic hydrogen.
  • Which Ka value is used when relating the first and second intermediate forms in a triprotic buffer?
    Ka2 is used when relating H2A⁻ and HA²⁻.
  • What is the fully deprotonated form of a triprotic acid?
    The fully deprotonated form is A³⁻.
  • When using the Henderson-Hasselbalch equation, what can be used in place of molarity for the ratio?
    Moles can be used in place of molarity for the ratio in the Henderson-Hasselbalch equation.
  • Why is it important to know which forms of a polyprotic acid are present when calculating pH?
    It determines which Ka value and which form of the Henderson-Hasselbalch equation to use.
  • What is the general formula for calculating moles from volume and molarity?
    Moles = liters × molarity.
  • In a buffer calculation, if you have 0.10 M citric acid and 0.15 M of its first intermediate, which Ka do you use?
    You use Ka1 because you are relating the acid form and the first intermediate.
  • What is the pH if you have 0.25 moles of the more basic form and 0.10 moles of the more acidic form, using Ka2 = 1.7 × 10⁻⁵?
    The pH is calculated as pH = -log(1.7 × 10⁻⁵) + log(0.25/0.10), which gives 4.94.
  • What should you always check before applying the Henderson-Hasselbalch equation to a polyprotic buffer?
    You should check which forms of the acid are present to select the correct Ka value.