When a pure monosaccharide of the alpha-D-pyranose form is placed in a plane pol

Question

When a pure monosaccharide of the alpha-D-pyranose form is placed in a plane polarimeter it rotates plane- polarized light +149.3 degrees. The pure beta-anomer of this monosaccharide rotates plane polarized light to +51.6 degrees. When the pure anomer of either form is left standing, an equilibrium is reached and the observed rotation is +78.5 degrees. What is the percentage of the alpha-form in the equilibrium mixture. Assume the contribution of linear or other cyclic forms is negligible, and report your answer as a percent to the nearest ones.

Explanation / Answer

Let x be % of alpha form.

So % of beta form = 1-x

From given data 149.3x + (1-x)51.6 = 78.5

x = 0.275 or 27.5 %

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