An edta solution was prepared by dissolving 3.7015g of disodium acid dihydrate (mw=372.3) in enough water to make 100ml of solution a .01732g portion of a solid sample which contained copper(ii) was dissolved in 100 ml of a ph 2 bufer solution. the endpoint of the titration was 9.98ml. calculate the percent copper in the solid sample.

The percent copper in the solid sample is 27.5 %

Explanation:

Step 1: The balanced equation

Cu + EDTA → Cu-EDTAcomplex

Step 2: Given data

mass of disodium acid dihydrate (EDTA) = 3.7015g

Molar mass of EDTA = 372.3 g/mol

volume of the solution = 100 mL

mass of the copper(II) containing sample = 0.01732g

pH of the solution = 2

The endpoint of the titration = 9.98mL

Step 3: Calculate number of moles of EDTA

Moles = mass of EDTA / Molar mass

moles = 3.7015g /372.3g/mol

moles = 0.00994 moles

Step 4: Calculate molarity of EDTA

Molarity of EDTA = number of moles / volume of EDTA

Molarity = 0.00994 moles / 0.1 L = 0.0994 M

Step 5: Calculate moles of Cu

moles = molarity * volume

moles = 0.0994 M * 0.00998L = 0.000992 moles

Step 6: Calculate mass of Cu

Mass of Cu = moles of Cu * Molar mass of Cu

mass of Cu = 0.000992 moles * 63.546 g/mol

mass of Cu = 0.063 grams

Step 7: Calculate the procent copper

(0.01732 grams/ 0.063 grams) = 27.5%

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

pre-sliced bread is a relatively recent development in baking technology. previous to that, it was always sold in a bakery in daily-made contiguous loaves with no wrapping, and was moderately perishable, usually being used with a day or two (which is why every bakery had a discounted “day-old” bread table.) therefore, pre-sliced bread would grow stale quickly. the hard crust provided a protective layer, and once the exposed dry slice left from the previous cut was removed, the interior was still soft.

what keeps bread soft is the moisture retained in the crumb (the light spongy part that’s not crust). the reason the crust develops is the surface of the dough gets exposed to hot dry air during baking which browns the outer layer and seals it, preventing all the moisture from escaping the dough. longer exposure to air will also eventually cause your bread to lose flavor and become stale.

people were used to cutting slices as they needed them and storing bread in a specially-made wooden or metal breadbox or pail with a lid to preserve it longer, but the first automatically sliced commercial loaves were produced on july 6, 1928, in chillicothe, missouri, using a machine invented by otto rohwedder, an iowa-born, missouri-based jeweler.[1] pre-sliced bread would indeed dry out, were it not for another 20th century technology: the plastic bag. bread could now be pre-sliced, and if the enclosing bag remains sealed as much as possible, the bread will remain soft and fresh for longer than if sitting unwrapped on a shelf or enclosed in a bread box that is not air-tight. sealed packaging meant bread could also be sold on store shelves and eliminate frequent trips to a bakery.

sealing bread in a plastic bag had another effect: the moisture in the loaf will re-distribute into the crust and soften it, which is sometimes more pleasing in a sandwich - especially for children whose teeth might not yet be strong enough to tear through a hard crust. a homemade loaf of bread can be sealed in a bag overnight and the crust will become soft like store-bought sandwich bread.

these were novel changes in how bread was produced and sold, which led to the coining of the phrase “the greatest thing since sliced bread.”