Vitamin , cyanocobalamin, is essential for human nutrition. it is concentrated in animal tissue but not in higher plants. although nutritional requirements for the vitamin are quite low, people who abstain completely from animal products may develop a deficiency anemia. cyanocobalamin is the form used in vitamin supplements. it contains 4.34% cobalt by mass. calculate the molar mass of cyanocobalamin, assuming that there is one atom of cobalt in every molecule of cyanocobalamin.

1360.0 g/mol

Explanation:

If there is one atom of cobalt in every molecule of cyanocobalamin its chemical formula must be

CoX₂, so there is 1 mol of cobalt for 1 mol of the compound. The molar mass of the cobalt is 59 g/mol. So, there is 59 g of cobalt in 1 mol of the cyanocobalamin and it represents 4.34% of the total mass, so:

0.0434m = 59

m = 1359.45 g of cyanocobalamin

The molar mass of the molecule is the mass divided by the number of moles (which is 1), so it is:

M = 1359.45 g/mol = 1360.0 g/mol

when 25.0 ml of 0.700 mol/l naoh was mixed in a calorimeter with 25.0 ml of 0.700 mol/l hcl, both initially at 20.0 °c, the temperature increased to 22.1 °c. the heat capacity of the calorimeter is 279 j/°c.

the equation for the reaction is:

naoh + hcl → nacl + h₂o

moles of hcl = 0.0250 l hcl ×

0.700

mol hcl

1 l hcl = 0.0175 mol hcl

volume of solution = (25.0 + 25.0) ml = 50.0 ml

mass of solution = 50.0 ml soln ×

1.00

ml soln

= 50.0 g soln

δt=t2-t1

= (22.1 – 20.0) °c = 2.1 °c

the heats involved are

heat from neutralization + heat to warm solution + heat to warm calorimeter = 0

q1+q2+q3=0

nδh+mcδt+cδt=0

0.0175 mol ×

δh

+ 50.0 g × 4.184 j·g⁻¹°c⁻¹ × 2.1 °c + 279 j°c⁻¹ × 2.1 °c = 0

0.0175 mol ×

δh

+ 439.32 j + 585.9 j = 0

0.0175 mol ×

δh

= -1025.22 j

δh=1025.22j0.0175 mol= -58 600 j/mol = -58.6 kj/mol

the answer is:  

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