The standard of oxalic acid is a known high purity substance that can be dissolved to produce a primary standard solution in a known volume of solvent. To prepare a certain quantity of oxalic acid, the respective known solvent weight is dissolved. It is ready using a standard, like a primary standard substance. Let us look at the preparation of standard oxalic acid solution briefly.
To prepare a standard solution of oxalic acid of M/10 or to prepare the standard solution of 0.1 m oxalic acid.
Hydrated oxalic acid => C2H2O4.2H2O
The molecular mass of Oxalic Acid => 126.
12.6 g of oxalic acid/liter of the solution should be dissolved in order to produce an M/10 oxalic acid solution.
On the other side,
12.6 /4 = 3.15 g of crystals of oxalic acid should be dissolved in water, and 250 ml of the solution should be produced precisely.
250ml measuring flask
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Take a watch glass and wash it with distilled water and later dry it.
Weigh the exact amount of dried and clean watch glass and record its weight in the notebook.
Weigh accurately on the watch glass 3.15 g of oxalic acid and note this weight in the notebook.
Transfer oxalic acid softly and carefully using a funnel from the watch glass into a clean and dry measuring flask.
Then, wash the watch glass using distilled water to move the particle position that sticks to it into the foam using the assistance of a wash bottle.
For this reason, the volume of distilled water should not exceed more than 50 ml.
Wash funnel as often with distilled water to move the position of the sticking particle into the measuring flask with a wash bottle. Add water in little quantities while washing the funnel. The distilled water quantity being used for this purpose should not exceed 50 ml.
By using a wash bottle, carefully wash the funnel with distilled water to pass the solution attached to the funnel into the measuring flask.
Turn the measurement flask until the oxalic acid gets dissolved.
Again, using a wash bottle, add enough distilled water thoroughly to the measuring flask just below the etched mark on it.
After that, add the remaining few ml of distilled water drop into the measuring flask until the reduced meniscus level touches the mark.
Keep the stopper on the mouth of the flask and shake softly to maintain the uniformity in the entire solution. Now, calculate it as a solution of oxalic acid M/10.
250cm3 of (M/10) solution or decimolar of oxalic acid is prepared.
The oxalic acid crystals need weights of 2g + 1g + 100mg + 5mg while weighing.
Wash the watch glass carefully, ensuring that not even a single crystal of oxalic acid is left on the watch glass.
Using a pipette, add the remaining few drops to avoid an extra addition of distilled water to the mark above the neck of the measuring cylinder.
If it is needed to titrate oxalic acid or oxalate, add the necessary dilute amount of H2SO4 and heat the flask at a range of 60° - 70° C.
If the solution is already prepared, then a few ml of this solution has to be titrated with known strength of base (NaOH) solution using an indicator or with KMnO4. Then, use the formula V1S1=V2S2 to calculate the strength.
Where V1 & V2 are the volumes of acid and base respectively, and S1 & S2 are the strength in the normality of acid and base, respectively. Now, divide the normality by 2 to get the molarity of oxalic acid.
If the solution is about to be prepared, then find the mole number of oxalic acid.
The molecular weight of hydrated oxalic acid (COOH)2.2H2O is 126.
So, mole number - m = weight taken ÷ 126.
Since the volume is 250 ml, 1/4 of 1000 ml, so the molarity = m×4.
Equivalent weight is used in the ratio and index calculations for various 2-component systems, such as epoxies and polyurethanes. Classically, it is defined as the molecular weight divided by functionality; that gives you a weight unit per reactive site essentially.
Suppose, you have a Side A having an equivalent weight of 135, and a Side B with an equivalent weight of 150. This means 135g of component A has the same number of reactive sites exactly as 150g of component B. And the ratio falls at 135:150. The ratio A: B, gives us a perfect stoichiometric balance (sometimes desirable and sometimes not).
In the case of a polyol, the equivalent weight is 56,100 divided by the ‘OH’ value. But, for isocyanates, it is 4,200, which is divided by the NCO weight percent.
Here, the epoxies are similar. For resin, it is the molecular weight divided by the number of epoxy groups. For the hardener/activator, it is the amine’s molecular weight divided by the number of active hydrogen atoms.
1. What are the terms gram equivalent and equivalent mass?
Gram equivalent mass is the mass of one mole of an element, ion, molecule divided by their valency or by the number of electrons shared.
For example, the molar mass of AlCl3 is 133.34 g/mol, whereas its equivalent mass would be 133.34/3= 44.4467 g/eq.
Defining the gram equivalent and the equivalent mass of either an atom, or molecule, or ion, it is a dimensionless number, but the equivalent mass has dimensions.
For AlCl3, the equivalent mass is 44.44 g/eq. Thus, 44.44 g of AlCl3 would mean 1 gram equivalent of AlCl3.
2. What is the Normality of a solution?
The Normality of a solution is described as the number of gram equivalents of solute per liter of the solution. It is a very common unit of concentration for acids, bases, although it is also used for other classes of compounds.
It is also related to Molarity as N=xM. Whereas N is the Normality, M is the molarity (number of moles of solute per liter of the solution), x is the n factor of the solute.
The n factor is defined differently for different classes of compounds. It is discussed below.
For acids, n factor is basicity. For example, the ‘n’ th factor for HCl is one because it gives one H+ ion. For H2SO4, the n factor is two since it will give two H+ ions.