Weighing by difference

While dispensing and weighing a small amount of the dried sample contained in the weighing bottle, it is prudent to avoid contacting the sample with laboratory utensils or weighing paper in order to avoid loss of a portion of the sample, which might adhere to the utensil or paper. In such instances, weighing by difference is recommended. 

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A round flask (250 c.c.) is fitted with an air-condenser. The phosphorous pentachlorlde is introduced from the bottle and weighed by difference. The operation must be conducted in the fume-cupboard. The benzoic acid is then added, and the air-condenser attached to the flask. The action begins almost immediately, and clouds of hydrochloric fumes are evolved. The whole contents become liquid and consist of benzoyl chloride (b. p. 200 ), phosphorous oxychloiide (b. p. 107°), and unchanged pentachloride. Most of the oxychloride may be removed by distilling in vacuo on the water-bath. The remainder is fractionated at the ordinary pressure and collected at 190-200°. Yield, 20 —25 giams. 

What is the advantage to weighing by difference, as opposed to weighing using a spatula and weighing paper ... 

During what period of the weighing-by-difference process must fingerprints on the item being weighed be avoided Explain. 

As with gravimetric analysis, the weight of the sample (the denominator in Equation (4.33)) is determined by direct measurement in the laboratory or by weighing by difference. The weight of the analyte in the sample is determined from the titration data via a stoichiometry calculation. As discussed previously, we calculate moles of substance titrated (in this case, the analyte) as in Equation (4.21) ... 

Prepare three 250-mL beakers and weigh, by difference, three samples of the unknown KHP, each weighing between 1.0 and 1.3 g, into the beakers. Dissolve in 75 mL of distilled water. Place watch glasses on the beakers. 

Powder the 20 paracetamol tablets and weigh by difference a quantity of tablet powder equivalent to 125 10 mg of paracetamol... 

Systematic error of balance (also known as sensitivity) For tared balances, or weighing by difference, as long as the masses are not too different any systematic effects cancel 0 mg... 

Preparative Technique.— Xenon difluoride and the involatile pentafluorides were weighed by difference into a Kel-F bottle. These operations were carried out in a Vacuum Atmospheres Corporation Dri-Lab, with a dry... 

Analytical four-figure balances for quantitative work, which weigh to the nearest 0.0001 g (0.1 mg) and have a maximum capacity of about 100 g. Chemicals must not be transferred onto the balance at any time and analytical balances must only be used for weighing by difference. 

Powder may be left on the tray, but weighing by difference does not solve the problem fully. The powder remaining on the tray will most likely be cohesive fines, leading to bias as a result of exaggerated concentrations of coarse particles analyzed. 

Remove your dried and cooled sample of cobalt coordination compound from the desiccator, and immediately weigh (by difference, using an analytical balance)... 

Using weighing-by-difference technique as you did in the spectrophotometric experiment, measure a 0.10-0.15 g (to the nearest 0.0001 g) sample of your coordination compound into a 250 ml beaker. 

Q2.37 Should your mass be measured using weighing-by-difference technique ... 

In all accurate pharmaceutical analyses, samples are weighed by difference that is, the weight of sample added to the flask is determined by subtraction of consecutive weighings of the sample container. The procedure adopted is as follows. 

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