Volume measurement balances

Previously, we discussed the need to know when a weight measurement does and does not need to be precise so that the appropriate balance is chosen for the measurement. We now repeat many of our previous comments, but for volume measurements rather than weight. 

The key to any reaction experiment is moles. The numbers of moles may be calculated from various measurements. A sample may be weighed on a balance to give the mass, and the moles calculated with the formula weight. Or the mass of a substance may be determined using a volume measurement combined with the density. The volume of a solution may be measured with a pipet, or calculated from the final and initial readings from a buret. This volume, along with the molarity, can be used to calculate the moles present. The volume, temperature, and pressure of a gas can be measured and used to calculate the moles of a gas. You must be extremely careful on the AP exam to distinguish between those values that you measure and those that you calculate. 

It is worthwhile to discuss the components of the standard uncertainty of a volume measurement here. The repeatability may be independently assessed by a series of fill-and-weigh experiments with water at a controlled temperature (and therefore density) using a balance so that the uncertainty of weighing is small compared with the variation in volume. Although this may be instructive, if the whole analysis is repeated, say, ten times, then the repeatability of the use of the pipette, or any other volume measurement is part of the repeatability of the overall measurement. This shows the benefit, in terms of reaching the final estimate of measurement uncertainty more quickly, of lumping together uncertainty components. 

Buoyancy corrections can in the present set-up be made by the in-situ measurement of the density of the gas by means of the weight change of a sinker with a known volume. This balance is described extensively by Dreisbach etal. [2]. 

This methods consists of adsorbing on surface atoms of the active phase (e.g. a metal) a molecule likely to give rise to a balanced chemical reaction. The volume measured at saturation converted to standard conditions, and corresponding to the formation of a complete mono-layer is used to determine the metallic surface the average size of the crystallites and the dispersion of the metal with the aid of the following equations ... 

Empty the beaker and dry it as well as possible with a paper towel. Place 4 mL of water in the beaker again, but this time measure this volume by adding the water from a 50-mL buret. Again measure the mass of the beaker on the three balances and subtract the mass of the empty beaker to obtain the mass of the water on each balance. Then pour the water into the graduated cylinder and record the volume reading. Be sure to record the volume measurement and all mass measurements in your Data section and clearly label all entries. Again you will have three sets of data for the mass measurements, one for each balance. 

To measure density, the weight of a known volume of the liquid is measured and this weight is then divided by the volume to obtain density. For the volume measurement, you will use either a 10-mL buret or a 10-mL graduated cylinder. Make sure the buret or cylinder to be used is as dry as possible or rinsed in the manner described at the beginning of this SOP. (A buret is a graduated cylinder with a stopcock valve at the bottom.) Then, weigh a small (dry) beaker on the balance provided. Record this weight in your notebook. 

An empty 2-L graduated cylinder is weighed on a balance and found to have a mass of 1124.2 g. Liquid methanol, CH3OH, is added to the cylinder, and its volume measured as 1.20 L. The total mass of the methanol and the cylinder is measured as 2073.9 g. Based on the way these data are reported, what do you assume is the range of possible values that each represents (See Section 1.5.)... 

Using a pipettor - check your technique (precision) by dispensing volumes of distilled water and weighing on a balance, assuming 1 mg = 1 gL = 1 mm3. For small volumes, measure several aliquots together, e.g. 10 x 15//L = 150 mg. Aim for accuracy of 1%. 

Thinking it Through Often chemical arithmetic is based not on a mass measurement, but on a volume measurement. The molarity of a solution expresses the number of moles of solute in a liter of solution. It is straightforward in this case to predict the volume of SO iaq) that will be required. The SO (aq) is halfes concentrated as the Ff (aq> solution, but only half Has number of moles are required according to the balanced equation. Therefore, 24.0 mL of the SO (aq) will be required for the titration to be completed. This is choice (B). 

All mass balances were obtained using the volumes and composition of input reactants and output products. Routine on- and off-line samplings were used to determine the concentrations of the intermediate species in the process streams. The inventory of materials in the system was estimated from the volume of material in each vessel. All streams leaving the process were analyzed and this, in conjunction with volume measurements, gave the total inventory of the species for the mass balance. 

The measurements chemists make are often used in calculations to obtain other related quantities. Different instruments enable us to measure a substance s properties The meter stick measures length or scale the buret, the pipet, the graduated cylinder, and the volumetric flask measure volume the balance measures mass and the thermometer measures temperature. Any measured quantity should always be written as a number with an appropriate unit. To say that the distance between New York and San Francisco by car along a certain route is 5166 is meaningless. We must specify that the distance is 5166 kilometers. The same is true in chemistry units are essential to stating measurements correctly. 

Using the right balance and the right volume measuring device ... 

Knowing the minimum weight (Mmm) that can be weighed on that balance and the minimum amount to be measured with the volume measuring device. 

Balances/scales and volume measurements, inertial mass, gravitational mass, huoyance, Coriolis force, electrical induction... 

In addition to the desire to understand this part of the physical world, there is another reason, a practical one, for studying the gas laws. This reason is concerned with the measurement of gases. The most convenient way to determine the amount of material in a sample of a solid is to weigh it on a balance. This can also be done conveniently for liquids or we may measure the volume of a sample of a liquid, and, if we want to know its weight, multiply the volume by its density, as found by a previous experiment. The method of weighing is, however, not conveniently used for gases, because their densities are very small volume measurements can be made much more accurately and easily by the use of containers of... 

Systematic error can be taken into account through calibration, comparing (he measuring device with a known standard. The systematic error in graph B, for example, might be caused by a poorly manufactured cylinder that reads 25.0 when it actually contains about 27 mL. If that cylinder had been calibrated, the student could have adjusted all volumes measured with it. The students also should calibrate the balance with standardized masses. 

Cost. Usually, the gravimetric technique is costlier than the volumetric technique. The volumetric technique requires only high precision pressure transducers and high precision volume measurements. The gravimetric, however, requires a high precision vacuum balance and, perhaps, considerable set-up effort. 

Mass balance evaluation was effected on exactly weighed samples by direct volume measurements (Method A) and by calculating the total particulate volume according to Nystrom (Method B). 

Reservoir engineers describe the relationship between the volume of fluids produced, the compressibility of the fluids and the reservoir pressure using material balance techniques. This approach treats the reservoir system like a tank, filled with oil, water, gas, and reservoir rock in the appropriate volumes, but without regard to the distribution of the fluids (i.e. the detailed movement of fluids inside the system). Material balance uses the PVT properties of the fluids described in Section 5.2.6, and accounts for the variations of fluid properties with pressure. The technique is firstly useful in predicting how reservoir pressure will respond to production. Secondly, material balance can be used to reduce uncertainty in volumetries by measuring reservoir pressure and cumulative production during the producing phase of the field life. An example of the simplest material balance equation for an oil reservoir above the bubble point will be shown In the next section. 

Reservoir pressure is measured in selected wells using either permanent or nonpermanent bottom hole pressure gauges or wireline tools in new wells (RFT, MDT, see Section 5.3.5) to determine the profile of the pressure depletion in the reservoir. The pressures indicate the continuity of the reservoir, and the connectivity of sand layers and are used in material balance calculations and in the reservoir simulation model to confirm the volume of the fluids in the reservoir and the natural influx of water from the aquifer. The following example shows an RFT pressure plot from a development well in a field which has been producing for some time. 

The automated pendant drop technique has been used as a film balance to study the surface tension of insoluble monolayers [75] (see Chapter IV). A motor-driven syringe allows changes in drop volume to study surface tension as a function of surface areas as in conventional film balance measurements. This approach is useful for materials available in limited quantities and it can be extended to study monolayers at liquid-liquid interfaces [76],... 

Neumann has adapted the pendant drop experiment (see Section II-7) to measure the surface pressure of insoluble monolayers [70]. By varying the droplet volume with a motor-driven syringe, they measure the surface pressure as a function of area in both expansion and compression. In tests with octadecanol monolayers, they found excellent agreement between axisymmetric drop shape analysis and a conventional film balance. Unlike the Wilhelmy plate and film balance, the pendant drop experiment can be readily adapted to studies in a pressure cell [70]. In studies of the rate dependence of the molecular area at collapse, Neumann and co-workers found more consistent and reproducible results with the actual area at collapse rather than that determined by conventional extrapolation to zero surface pressure [71]. The collapse pressure and shape of the pressure-area isotherm change with the compression rate [72]. 

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